WARNING: We're playing through Mines of Phandelver... this post contains obtuse spoiler references - so if you don't want to know, don't read on! You have been warned.
So my new wizard just broke down and destroyed Iarno Albrek! We chased him half-way through the dungeon, but we got him. And the Cleric cold-cocked him so we could interrogate him. My son, the new DM, said the adventure didn't say he had a spellbook. How odd. So he decided like any good wizard, he had it on his person.
So now I have his spellbook (and the glass staff!)! What? 50 gp to copy Charm Person, 100 gp to copy Hold Person, and ANOTHER 100 gp to copy Misty Step? Are you kidding me?
I'm just going to carry his book with mine and memorize the spells straight out of his book! Oops, did I just break the game (and save myself 250 gp!)??
I'm a grizzled old D&D player from 1st Ed. My son is amazed at my tactics, my trickery, and my ability to put him in difficult DM judgment situations. >:-) For example flying my familiar owl through the Cragmaw Hideout to see where all the guards were hiding really ticked him off.
P.S. (How do you create links in your post to spells and stuff in your post? Adding a URL or something else?)
You can't prepare spells from someone else's spellbook because 1) you haven't practiced the spell and 2) it's not written in a form you understand. Each wizard has their own notation for describing spells, so until you figure out what the pages in Albrek's spellbook mean, practice the spell until you get it right, and write down what you learned in your own notation, you can't prepare the spell. The material components you use up figuring out and practicing the spell factor into the cost of adding spells to your spellbook.
I always found the 5e restriction on reading another wizard's spellbook to be weird and instead follow earlier editions which allow a wizard to figure out the notations in captured spellbooks and prepare spells from them directly.
The cost is not just the ink, but the cost of basic materials to experiment and learn the spell. Just because I can read it, doesn't mean I know it. You can look at the spellbook as the same as little limericks you learn in grade school. "Please excuse my dear aunt sally" for Parenthesis, Exponents, Multiplication, Division, Addition, Subtraction or remembering the directions on a compass as clockwise "Never eat shredded wheat" for North East South West, or a personal favorite "Lefty Loosey, Righty Tighty." These are all different ways to learn universal truths. Your spell book is the "Left Loosey, Righty Tighty" of spells to help you memorize it faster. Someone else might learn "Clockwise down, Counter clockwise up" and it takes time to translate their shorthand to yours for the sake of RAPID memorization.
You cannot learn complicated formulas overnight if those formulas are not shorthanded in the way you learn spells.