Find similar genre games to your own game. See their prices. Put down the range to have some sort of understanding of the current market.
Ask feedback from Beta testers "How much are they willing to pay for such game?"
You can make a quick vid of your game and make a survey for online users (or friends), but if you go for friend route - your statistics will be unpredictable and not reliable due to bias.
If you are a fan to play games yourself - you can analyze yourself what kind of games were not worth their price.
Ask yourself "For how much am I willing to buy such game?"