To get started, you might want to work for free on a project that doesn't have any money anyway.
Then once you've worked on at least one game, you should charge what you can get (what you can negotiate).
You won't get paid as much as a professional would. You can read about game salaries (not the same as freelance pay, but it gives you numbers to work from) at
http://www.gamecareerguide.com/featu...ry_survey_.php
Also read this:
http://www.gamedev.net/topic/616744-pricing-question/
and especially this:
http://www.gamedev.net/topic/615716-...e-for-artists/