Depends on the morph, theres several red/blue combinations some more exotic than the next. Secondly, fair in terms of retail? Or a local hobbyist sell/trade?
Id say for a local reefer 1-20$ a polyp is fair.
For a retail outlet id think 15-30$ is a fair polyp range, depending on how common the morph actually is. Some of the most common and accessable micros are red and blue ones.