Varies from store to store here in my area - on average, they trade owners for something between 5 and 10 euro's (7,50 and 13 dollars US).
I bought mine for 3,50 euro (5 bucks) a piece, but they were about 1" in size, and that was 2,5 years ago.
What's reasonable in one area is a rip-off in other area's - price depends on supply and demand. If you live in an area close to large importers or heaps of fish enthousiasts, the prices will be lower than in area's where everything more fancy than a goldfish needs to be flown in...
But in the end there's no right or wrong price - if you're willing to pay a certain price for a certain fish, than it's fair enough.