Spellbooks have, generally speaking, 100 pages. It costs 100gp per page to scribe a spell.
It therefore costs, at a minimum, 10,000gp to scribe a full spellbook. By using the standard D&D half-price rules, this translates into 5,000gp.
This does not include the cost of procuring the spells that went into a spellbook, however, which can increase the cost to make it by a lot. The problem is that this is highly variable: a wizard's initial spell loadout doesn't cost him anything, scrolls found on adventure and spells copied from a friend cost only the scribing charge, spells copied from an other wizard's spellbook generally cost 25gp-50gp per level, etc.