I was suggesting prices for all 2-3 inch fish, but yes I would have to agree if we are suggesting applicable prices for 6-8 inches then you are definitely right and you can be looking at 2-3 times what my suggested prices were.

This one was atleast 5-7 inches he wasn't looking to great thought But I think the price was 89.99
Probably a scomb then. If its either of the other two types, it's a good deal.
I've seen one Armatus in an LFS in FL, he was about 12" and $500. I guess since nobody gets Armatus anywhere here, they like to over-price their fish. Apparently they were shipping him to somebody, but the buyer backed off, they were stuck with a foot long Armatus in a 30g holding tank.
If its clean, $500 for a foot long armatus is a little pricey by still fair. Keep in mind buying the same fish over the internet for $375-$400 plus shipping would likely cost you more than the LFS is asking.
I really do believe this to be the key to keeping healthy payara and when I do finally get one it will be kept on live. Although it is expensive I feel like many here will agree that they simply aren't getting something they need when we feed prepared foods or pellets. DB junkie will also probably tell you he feels the same as he hypothesizes this because his were often the first to be weened off live in favor of prepared foods and was often the first to pass.
Some of the larger specimens we have seen 24" and 27"+ specimens are often only seen in asia where mostly these fish have only been kept on feeders and goldfish.
Like I said previously Armatus seem to be a bit of a wild card, there are cases of people getting them much cheaper whether being because they come in through shipments of Scomb or Tats and are mistaken, or simply because the store has no idea what they have. However like the one locally to you have been seen for exorbitant amounts of money.
I feel live food is best for a predator, if you can provide the right kind of food. For a payara, this would be characins like silver dollars, small catfish and cichlids. Cyprinids like goldfish are not found in the same geographical area as payaras and are not a natural prey item. Theres also the thiaminase thing.
I agree you can get the 'better' payaras for a steal sometimes. I myself got my 7" tat from a shipment of scombs for just $40. At the same time, all of the tats of similar size on the site were going for ~$200.