How long is a piece of string?
There are nearly a limitless number of variables to consider...
Is game development your sole occupation? (Though the fact your asking about rates implies you're not)
What's the game? (A single level of Tetris is far more valuable than a single level of Mario)
Who's the client? (Rich entrepreneur or a idealistic student living on noodles?)
What platform is it for? Is it multi-plat?
Do you need to purchase additional assets/expenses? (Sound effects/music, developer licenses, testing hardware)
What's your experience level? (A 10 year vet will have a much better wage than year old freelancer)
...And just off the top of my head...
A good rule of thumb is:
Business costs + personal costs / hours billable = Break even rate
Then simply mark-up that figure at your leisure.