Usually i spend around $300/ semester, but so far I am lucky and only have to buy books for 3 classes. So this semester i spent about $200.
$150 for 3 engineering books
I recommend you guys check out Chegg.com - Rent Textbooks, Cheap Used College Textbooks, Save Big and be Green.
You rent the books for like a 1/4 of what you pay for at the bookstores and you don't have to worry about selling them back, just ship em back to the company at the end of the semester. They provide you with a prepaid waybill for all the books, just toss em all in a box