Do you mean
?
I'd expect them to be a bit more expensive than Pimelodus pictus of similar size, so maybe around $25-50 each. I did a quick search on the web, and not much came up, so I don't expect them to be FREQUENTLY available.
I did find one English web-site that lists P. ornatus,
http://www.trimar.co.uk, and they are usually competetively priced.
At £16.75 it makes around $30 each - but there's no size listed, so I expect them to be fairly small, 2-3" SL perhaps. Which would put the price for a 4" one a bit higher than that...
As mentioned, prices vary depending on where you live, what shop you go to, and what the wholesalers currently have available... Some fish are only available some part of the year, then it's a case of waiting for the next batch to be caught - of course, there's always someone around that want's to make a buck on those who can't wait for something to be "common" again. Just like some vegetables are much less expensive when they are in season locally than when they have to be shipped from the other side of the world...
One of the problems here is of course that if you actually want a particular fish from a local shop, it's no good saying "But you can get them in England for $30, so why are you asking $60?" - If I owned the shop, my reply would be, "Go ahead, get them from England then..." - local shops are LOCAL shops, and when we're dealing with live-fish, they aren't exactly like computer chips or books that don't matter where you buy them, they'll just sit on a plane or a lorry for a bit longer. Fish die when shipped - shipping them a long way will increase the risk of them dying. If they are shipped properly, the risk is less than if they are shipped by someone who doesn't know what he/she's doing, and the shorter the trip, the better...
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Mats