My girlfriend and I got a cat from a shelter about a month ago. She works full-time and is home less than I am, so my situation is probably similar to what yours would be. Since cats are pretty low maintenance and relatively anti-social, you'd have plenty of time to care for a cat. If you feel bad about them being alone all day, you could get two and they can keep each other company for essentially no more work.
As far as time involved for school, it'll depend very much on your goals and expectations. I'm at a P/F school and have no problems learning just what's in the board review books, so I don't do as well on exams but also don't study as much as some of my classmates. I also don't set foot in a classroom unless required, which again sets aside more free time. If you're going to go to lecture and try and score 90+ on your exams you're probably not going to have that much free time (unless you're a genius).
As far as cost, cats are very cheap (assuming no medical problems). The initial "setup" cost us about $150 total, and now we probably spend no more than $40-50/mo for food (we buy Science Diet and other more "premium" brands), snacks, and random toys for her. You should be able to spend anywhere from $500-1000 if something should happen, though. I think the cost is pretty much nothing considering what you get out of it.
...for even the mind depends so greatly on the temperament and on the disposition of the organs of the body that, if it is possible to find some means to render men generally more wise and more adroit than they have been up until now, I believe that one should look for it in medicine.
Rene Descartes, Discourse on Method