In terms of cost, it really depends upon where you live, and what services you require (essentially, how far up the lawyer food chain you need to go). I'd budget up to a grand to review an employment contract, for example.
As for how a student could afford one.... Maybe save the cash in advance, if you can, if you're going into a deal like this? Ask for help from friends and / or family? Look around for organisations offering legal assistance to students (e.g. http://www.lslap.bc.ca/)?