You can totally copy the spells and then sell the spellbook. The cost might vary based on the buyer and how common the spells are.
A 1st level wizard with only a handful of spells might pay quite a lot of money (assuming he has plenty of it) for a book with a number of 1st and 2nd level spells. That same spellbook might be worth very little to a wizard of 5th level or high who might only find a couple of additional spells. A book containing high level spells, especially those of 6th level or higher will probably always go for a high amount. How much is probably campaign dependent.