cheapest: free goldfish at those carnival games.
most expensive: $80 for a midas blenny. but if you want freshwater, I think I paid $7 for a praecox rainbow.
$80?! For a midas blenny? Never seen one that expensive. My lfs has some 4-5 inchers in right now for $45 (and they are usually overpriced).
Yes my LFS is horrible with prices. It was fully grown though, and they'd had it for a few weeks. It was so cute!

Plus the alternatuve would be buying online, and then with shipping it would just be the same.