This is going to sound snarky, but it's quick and accurate:
1) Bid on free agents to the amount you think reasonable.
2) If you find yourself bowing out early on power hitters because they're getting contracts you think are insane, power is overpriced.
Or you can use player search function, sort the results by power, and see where the contracts for guys with more than 6 years MLB experience seem to fall. This is less accurate because all players sign with their club as FAs the first go-round if the club wants them to.