Price of filters depends on brand, size, and type of coating.
Multi-coated one from B&W at 77mm would cost you quite a bit where as non-coated one from Tiffen at 52mm can be had for ten bucks or sometimes even less.
I took a middle ground and bought a set made by Hoya, multi-coated, and 77mm for about $50 each. I use adapter rings to mount this to smaller lenses.
I also have a set from Cokin where there is a bracket and I can slide in various square resin filters. I found these to be a pain to use, so I don't use them anymore.
If you have access to a set already, I'm not sure if I'd invest in my own. Unless they are in very bad shape, filter quality does not affect the image quality all that much, if any. You might want to play around with your school's filter for a while and get a feel for your own needs first.