They're not that expensive. I'm a grad student, and I have about 10 CDs myself. Go around the brick and mortar stores and places like soundsonline.com, and you can sometimes find some deals. Also, older libraries tend to be discounted...I got my BBC sound library cds for around 40 bucks a disc. Just looking at sound-ideas.com, there are more than a few libraries that are selling at 75 bucks a disc.
Also, if you don't need whole CDs, there are web sites that sell royalty-free sound effects a la carte for something like a buck to a few bucks a sound. Never used them before, but they seem interesting...