THe length is pretty irrelevant on the pricing of quality flowerhorns, globally. It's much, much more about the patterning, colour and size of kok (pardon the expression).
Unfortunately, The UK market is often rather ignorant of such subtleties, as the flowerhorn market is not huge here.
So of course when it comes to what price a shop "guesses" at what it will try and sell one at, it's almost like picking a number out of the air.
WoW, Manchester are the biggest FH retailer in the UK, and they will try and sell a FH for, say, £400 which might only go for $120 in the US, and only $40 in Singapore or somewhere, where they are very into their FH's, and where most of the global FH stock is bred.