Eureka! It is this simple:
- Every predictor gives two prices in log scale eg. "In 2014-5-16 the price is between 2.7 and 2.85 (roughly 500 and 700)"
- When the actual price is known, you take
min [ abs ( actual - upper_limit); abs ( actual - lower_limit) ]- Whoever has the lowest average error after a reasonable number of predictions (predictions can be renewed as often as you wish regardless of their maturity)
is the best! 
- Proof omitted

I would be very grateful if you could explain this to a simpleton like myself.