F/W may not be that interesting to a S/W diver, unfortunate that you are missing out though. But, no, the $ gain in the F/W is not as good as in S/W so I can see what you are saying.
In reality though, and though I do not keep any F/W at the moment, F/W have their own little niche in the aquarium hobby. Watching eartheaters breed and care for their young, tank raised discus of all different color morphs interact with each other and their owner, a shoal or rosie barbs streaming by lush green vegitation and even a tank full of African mouthbrooders with colors of such diversity and personalities to match...
Yes, S/W fish are costly and you can make a goodly amount collecting and selling them, but F/W have their place and are just as facinating, in some ways more so, especially if you are wanting to observe the breeding behaviors of tropical fish.
Please don't belittle the F/W aquarium species for they in themselves can bring enjoyment and awe to the keeper, and that so without having to fork out $600 for a fish that will never spawn in captivity and if it does happen to, rearing the larvae would be next to impossible.
But, for some the $ sign is all that matters.
And before anyone gets all bent out of shape becasue I spoke the truth, if the F/W species demanded a higher price than the S/W, where would you be? Diving off the reefs or standing in the streams with net in hand?