Mostly it depends on how long the game takes me to complete. I judge the value (in dollars) of a game based on how long it keeps me entertained. And quality does play a role in this, because I would be willing to give a good game a second and possibly third playthrough, thus extending the amount of time I have been entertained by it.
A single playthrough of Mass Effect 2 took me 30 hours. I would be perfectly fine paying $60 American for that, especially considering I plan to play it again. Call of Duty 12 (or whatever they are now on) would take me about 5 hours to complete, and I do not fancy the multiplayer much. I would not pay much for that game, maybe $10-15 American.
Considering all games are different, though, it's kinda hard to set a universal price for them.
To answer the question, I guess $40-45 American would be a fair price for a game I have never played.