Quote from sewerrat78 >>

I have a question regarding copying spells that you already know from one spellbook to another.  Why is there a gold cost for the ink?  I understand the cost for using materials to learn the spell, but why do you need a specific ink?  The first answer I get is because of the magic in the ink, but there is no magic in the ink or anywhere in the spellbook (unless you're order of scribes and have an awakened spellbook). If there was magic in the ink I think that would be some type of rune magic.  The other answer I get is that you need to get a high quality ink that doesn't splotch, blot, or smear.  This doesn't make sense to me either as splotching and blotting comes from a poor quality pen-not ink. Smearing comes from trying to hurry the job and smudging the ink with your hand before it dries.  Most times when the ink becomes unreadable it is because the paper was damaged.  Even when paper gets wet but doesn't dissolve the ink is still readable.  It makes more sense to me to require a specific high quality pen that allows you do write fine lines without splotching.  It also makes more sense to require specific paper which would be more resistant to environmental hazards.  Am I just missing something?

 
This is like scanning a page and going "why do I need ink to print it?"

You need ink because you're using more ink onto a new page.

The ink isn't magical. It's just high quality and is in line with the actual cost of "ink" as detailed in PHB section on mundane gear. 

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But sure, let's break it down.

Copying a new spell:

Copying a Spell into the Book. When you find a wizard spell of 1st level or higher, you can add it to your spellbook if it is of a spell level you can prepare and if you can spare the time to decipher and copy it.

Copying that spell into your spellbook involves reproducing the basic form of the spell, then deciphering the unique system of notation used by the wizard who wrote it. You must practice the spell until you understand the sounds or gestures required, then transcribe it into your spellbook using your own notation.

For each level of the spell, the process takes 2 hours and costs 50 gp. The cost represents material components you expend as you experiment with the spell to master it, as well as the fine inks you need to record it. Once you have spent this time and money, you can prepare the spell just like your other spells.

Let's simplify this.

The steps detailed are 

1. Reproduce basic form of spell
2. Decipher the original notation used
3. Practice spell.
4. Transcribe spell using your own notation.

The costs are:

Time. 2 hours per spell level.
Money: 50 gp per spell level which is broken down as 2 thinks: 
- Money A - material components you expend as you experiment with the spell to master it
- Money B - the fine inks you need to record it

So, now lets compare to re-scribing your own spell. It doesn't matter if it is from memory or book to book : your spell, going into new book:

Replacing the Book. You can copy a spell from your own spellbook into another book—for example, if you want to make a backup copy of your spellbook. This is just like copying a new spell into your spellbook, but faster and easier, since you understand your own notation and already know how to cast the spell. You need spend only 1 hour and 10 gp for each level of the copied spell.

If you lose your spellbook, you can use the same procedure to transcribe the spells that you have prepared into a new spellbook. Filling out the remainder of your spellbook requires you to find new spells to do so, as normal. For this reason, many wizards keep backup spellbooks in a safe place.

The costs here are just 1 hour and 10 gp per spell level. It mentions that the reduction is due to already knowing the spell and notation. SO

Steps 1 to 3 are gone, as is Money A.

So this means Steps 1 to 3 is worth 1 hour per level, so with that gone the actual scribing of the spell is just 1 hour per spell level. Since Money A is gone then Money B is 10 gp per level of spell.

So, of the 50 GP original cost, only 10 gp is for the ink.

And wouldn'tcha know it, there in PHB:

So the 10 gp Ink you need is just the normal price for normal ink. You're just using a whole bottle of the stuff for each level of the spell. Which seems like a lot but it really isn't. That 1 ounce can go quick, 'specially with the calligraphy, magic symbols and runes and notes and whatnot. 

Hope that clarifies things. 

I wasn't asking "why ink."  I was asking why a "fine ink."  You could have said, "You're just using regular ink and the PHB says 1oz of ink costs 10gp."  That would have settled things for me.

But your explanation has actually given me more questions.

Where is all that ink going?   One oz of ink can fill a few hundred pages with text.  This indicates that it would take 1600 pages to write down a 4th level spell.  I realize that modern ink is of much higher quality than you would probably find in a fantasy setting, even so, that is a lot of ink.

Also, you said, "That 1 ounce can go quick, 'specially with the calligraphy, magic symbols and runes and notes and whatnot."

Why calligraphy? Why do I need fancy penmanship?  Why do I need big stylized letters and pictures to show that the sound I make needs to sound like the ch in 'champagne' and not 'cheese'?  Or to show how big that diamond need to be so I can cast Chromatic Orb?

Why magic symbols and runes?  Magic symbols and runes imply that there is magic in the text, which both you and I said was not the case.  Why do they need to be in the spellbook if they aren't actually part of the spell?

Forgive me if I sound ignorant or argumentative.  I'm not trying to be, it's just that part of the answer still doesn't make much sense.

Quote from sewerrat78 >>

I wasn't asking "why ink."  I was asking why a "fine ink."  You could have said, "You're just using regular ink and the PHB says 1oz of ink costs 10gp."  That would have settled things for me.

But your explanation has actually given me more questions.

Where is all that ink going?   One oz of ink can fill a few hundred pages with text.  This indicates that it would take 1600 pages to write down a 4th level spell.  I realize that modern ink is of much higher quality than you would probably find in a fantasy setting, even so, that is a lot of ink.

Also, you said, "That 1 ounce can go quick, 'specially with the calligraphy, magic symbols and runes and notes and whatnot."

Why calligraphy? Why do I need fancy penmanship?  Why do I need big stylized letters and pictures to show that the sound I make needs to sound like the ch in 'champagne' and not 'cheese'?  Or to show how big that diamond need to be so I can cast Chromatic Orb?

Why magic symbols and runes?  Magic symbols and runes imply that there is magic in the text, which both you and I said was not the case.  Why do they need to be in the spellbook if they aren't actually part of the spell?

Forgive me if I sound ignorant or argumentative.  I'm not trying to be, it's just that part of the answer still doesn't make much sense.

 
At the end of the day "because the designers said so".

Rationalise it however you want.

For me, a spell could include detailed layering of glyphs upon glyphs - patterns of the weave one must study and fix into the mind, along with additional notes regarding sounds and gestures, list of effects, etc. 

The designers didn't go into detail about it because for the vast majority of players it doesn't make a difference. There's a lot of this in D&D. It's fantasy, it's magic and the world is based on the "weave of magic" not in physics. 

The designers themselves can't give you an answer you're going to find satisfying. Nobody else who replies here is going to be able to do that. The cost breakdown has been given. The rules has been stated. Rationalise it how you need to or talk to the DM about a houserule. ¯\_(ツ)_/¯

I appreciate your time and your help.  I guess my problem is that I can't rationalize it.  I can't make it make sense. For every other question I've had about mechanics I've been able to find a reasonable answer.  Humans have a base walking speed of 30 feet because 5 feet per second equals 1 mile per about 17&1/2 minutes, and most adult humans comfortably walk a mile in between 15 and 20 minutes.  Owls have Flyby because in actuality owls fly silently.  Even more fantastical aspects, such as spellcasting, have a basis in reality.  You need bat guano and sulfur to cast Fireball because you can use bat dung (or rather the potassium nitrate that is in bat dung) and sulfur to create a ball of fire.  Add into it the fact that Wizards aren't necessarily magical creatures.  They've merely studied magic and figured out, for lack of a better word, the science behind it and build magic effects from the ground up.  It just seems that ink for magic in D&D would have similar reasoning.  If the only answer is because that's what the rules say, then I guess there I have it. 

Again, I wasn't trying to be argumentative and I appreciate your help.  No, you haven't been able to fully answer my questions, but you have given me something more to consider.  Thank you.

Why does a bowl of pea soup cost 170 Gil in Final Fantasy? Because it ain’t real life. Nuff said.

Just going through this thread and wanted some thoughts on scribing a spell from your spell book onto a scroll? I would expect that would be fairly easy, as you would already know the intricacies of your own spell. So would that mean it would be a reduced time and cost to do this? If my Wizard is about to depart on an adventure, I would want to scribe a few emergency backup scrolls. So, if I wanted to scribe a fireball scroll and I already have it in my spell book, what would be the time/cost? Would it be half the time as normal, or even less?

Quote from Jimbolicious >>

Just going through this thread and wanted some thoughts on scribing a spell from your spell book onto a scroll? I would expect that would be fairly easy, as you would already know the intricacies of your own spell. So would that mean it would be a reduced time and cost to do this? If my Wizard is about to depart on an adventure, I would want to scribe a few emergency backup scrolls. So, if I wanted to scribe a fireball scroll and I already have it in my spell book, what would be the time/cost? Would it be half the time as normal, or even less?

 
The rules for creating a spell scroll are in Xanathar's Guide to Everything.