So, I have a wizard who has recently come into a lot of money. Because of the way my campaign is structured, they won't level up and be able to learn level 3 spells until they're out of an environment where they have easy access to them.

So I'm kicking around the idea of just letting them buy a spellbook that has the complete set of level 1-3 wizard spells in it- for a substantial sum of money, of course! They could then promote this to their primary spellbook when they level up. (I know this isn't actually quite how spellbooks work, but I'm willing to futz it this one time. )

So I'd like to know:

1. Is this actually a totally insane idea? And

2. If not, how much should I be thinking of charging for it?

I would charge them twice what it costs for the basic spell book components. So 100 GP per spell level. 

And you’re nicer than I am. I would still make them spend another 50 GP per spell level to copy those spells into their spell book. 

+1 to lyxen.

Also, giving them all the spells seems like a bit much. Flexibility is really one of the stronger parts of the class, but restraining that a bit by only allowing a couple spells helps bring them back down to earth. I realize this won’t change how many spells they can cast a day, but it seems really strong.

Wizards aren't supposed to have all the spells. It's not just a balance thing, it's a matter of making choices that define the character and the role they want to play. It gives meaning to the choices you make.

"You get all the spells" also sets an expectation going forward that they will find something similar at higher levels.

It's fun to shower the party with loot, but you might be surprised how quickly that rush fades. All it ultimately does is set the bar higher and higher for future loot while you have to work increasingly hard to challenge the party. Whatever gold haul they just got is going to make any future hoard feel anticlimactic unless it's even bigger. You can have too much of a good thing - scarcity creates value. I would not do this.

+1 to Lyxen here as well.

It sounds a little bit to me (if I've read and understood you correctly) that you are foreseeing a problem and are trying to solve it in-game (how does the wizard get access to lvl 3 spells when he levels up). I would never have solved that issue that way. It is your world and your game. You decide how the wizard get new spells in his book. I would probably have solved it "off screen" in a meta way. For instance allow him to buy a spell book, but wait with deciding which spells are in it until he levels up so he could still choose that when he levels up in the middle of nowhere.

By just giving him a spell book with all the level 1-3 spells, you (in my opinion) chooses the least interesting road. You can just as well just say that in your campaign there are no spell books, wizards have access to all spells they can cast at their level. Period. One of the things that can work with a spellbook is that finding an interesting spell and writing it into the book is a nice reward to the player. If you make finding spells to hard, the player will get frustrated, but if you make it to easy (like I think you are doing), you also take away the satisfaction of finding a spell.

Another alternative is to give them a spellbook with several spells in, but it acts like a normal spellbook in that your wizard will have to spend the time and money to translate each spell that they want into their own spellbook. So unless the wizard has a LOT of time, he won't instantly have access to all the spells.

But I do agree with Lyxen that you shouldn't be penalising just this one class. Why aren't they able, over the course of the level, to work out the complex magical incantations required to learn their 2 free spells each level?

I would make it a multi-volume set of spell books.  A Single volume for the first level spells, a second for level two and a third for level three would make a lot of sense.  As a theme in your campaign you could have these books all have a particular color binding and mysterious moonglow lettering on the covers.  Heck, the whole book could be in moon glow runes, or maybe the lettering only appears in a magical light.  Then, every time your party runs across a "blank book" they start to wonder what secrets might be hidden within.