Usually fair price is based on some combination of 1) market price and 2) the investment's expected value, adjusted by some return hurdle rate. For example, if

Usually fair price is based on some combination of 1) market price and 2) the investment's expected value, adjusted by some return hurdle rate. For example, if I'm looking for a 25% annual return, and next year I think Amazon is 70% likely to be $1000/share and 30% likely to be $3000/share (weighted avg = $1600/share), then a fair price might be $1280.