First, see if there is a list of approved calculators. Second, see if, as it's required, you can get the school to supply you with one (I'm assuming you have a "technology fee" at your college. Find out what you paid for) or give you coupons.
There's
TI's Big PDF Comparison Chart for you to look at. Since they seem to be abandoning the 85/86 product line, you might be able to pick one of those up cheap. I have an 86 and have no complaints. If I were going to buy one now, I'd probably get an 84+AgEd or an 89Ti, depending on price.