Anything is only worth a price you can find a buyer willign to pay...
So who are your potential buyers?
A local pet shop will not be willing to pay "top dollar" as they have to house & feed the fish plus make enough profit to contribute to overhead costs plus put some actual profit in the owners pocket.
If you are willing and able to ship, they you have a nation wide hobbyist audience you are selling to. But if you cannot ship then you have a city wide potenial audience.
Then of course you have to consider the competition in your selling area. If you live somewhere that has 6" Blues for $40 at the store, then you'll not be able to get more than that. But if you live in a area where no one has ever seen a real life 6" Blue, you may be able to get well over $100 for him...