Fair price is anything buyers are willing to pay. Do you know the lineage of each fish and how do the offspring from this pair look? If you know the lineage and each parent comes from a stable line of quality fish, you can ask for more money.
If their offspring is high quality, you should add a picture of a higher quality offspring and then you can ask for more.
Not knowing the background on these fish, I would say $300 for the pair on the high end and $150 for the pair on the low end. Proven mated pairs usually cost more because often times, fish are incompatible and fight and get injured during courtship.
Simon