Mattress Toppers

My daughter is leaving for college very soon. One of the items suggested is a mattress topper. When I was in college I didn't get one of these, but they suggest it as the mattresses are not the most comfty.

Damn these things can be way over $100 or as low as $20. Anyone have thoughts on these - which are good or at least good enough for a college student. I've been reading some reviews - I also heard something about them being a fire hazard?

Then I read about foam ones, form fitting, and then another type. Any thoughts?