Interesting point of view, Archade. I'm not sure I agree that the value of a spellbook should be that high, but it brings up the idea of the "Found Spellbook as a Backup Copy". Unfortunately, this idea is fraught with problems too.
For example, I'm fresh out of Wizard school and I've got my spellbook with all the cantrips and (for sake of our exampel) five 1st level spells. So, I go out and run into Evil Eddie, another first level Wizard and I kill him and take his spellbook. Unfortunately, it doesn't have a single new spell in it and is, in fact, identical to my own spellbook. Bummer. It has no contributory value in terms of me gaining greater access to more spells. BUT, I take the book and leave it in my library at home. Now if I lose my original spellbook (prior to scribing anything else in it), I have a backup that contains every spell I know. If I had to create this by myself, it would have cost me 1/2 the normal scribing cost to make a backup copy.
Now, I go out and find Evil Elvis, Eddie's brother (and another 1st level Wizard) and kill him too. I get his spellbook and find that while it has the same cantrips I'm familiar with, all five of the 1st level spells in it are different than the ones in my book. This means that it has considerable contributory value toward a greater spell diversity, but I need 1,000gp to take advantage of this resource. Since I don't have 1,000gp handy to scribe them at the moment, I opt to stash it back at the house. But now if I lose my original spellbook and I go back to the house and grab this one, it is nearly worthless as a backup copy because the only spells in it that were in my original spellbook are the cantrips.
So that means that if I want to be able to cast Burning Hands, which is in Elvis's book but wasn't in my original book, I could copy it from page 1 to page 50 of the same book and now I can use it.

I know that most people are fine with these rules as written, but they just seem to invite headache.