The price for length really depends very much on the genre of game. The best way to determine this when making a game is to look up popular games that are similar and see what they are selling for and how long they are.
For adventure games (what I usually play and make), the going rate at the moment seems to be about $2-$3 per hour of gameplay. For RPG's however, that may have a longer play time but only because of repetitive battles (grinding) taking up a lot of the time, the cost per hour is going to be much less than that.
It also highly depends on the quality of the game as well. It also depends on how well-known the studio is. A well-respected studio known for quality games can charge more and still get enough sales. It also gets cheaper per hour the longer the game is, too. If you make an adventure game that's 50 hours long, that doesn't mean you can charge $100 for it because no one is going to pay $100 for an adventure game.
There are a lot of factors that come into this, but as I started off saying, the best way to find out a price for your game is to look for similar games of similar lengths that are doing well, and see what they're priced at.