if you dont care about platform, Dell Axim has a low end model with pretty good reviews. PPC platform.
the zire 31 is a lower end palm pilot, though now palmone.com is selling refurbished zire 71s for pretty cheap. But you may not want a palm that someone else returned.
I think the 31 is the lowest color zire, but i bet you can find some old models that palmone doesn't make any longer on ebay or other tech resale sites.
clie may have some priced at the 150 range.
HP should too, but it will probably be stripped down.
I would suggest you read all the PDA reviews on this site, and then give a more detailed request.
But as a student, i think you will probably need only the most basic utilities, plus a word processor.
For PPC thats Pocket Word,
For Palm thats Documents to Go.