as an ex-lfs employee I can say that seeing the trade price in comparrison to the retail was quite an eye-opener.
however once you factor in shipping, mortality rates, treatments (if required), staffing cost, running costs etc
the actual profit made on a fish sale is minimal. some profit in a LFS is made from dry goods (tanks, filters etc)
but most is made on consumables, like food, filter media etc.
and don't forget the final sale price has 17.5% taken away by the taxman (at least here in the UK it is 17.5%)