It's a Red Tail Golden (RTG). A reasonably priced 5" one would be around $450 to $650. The price doesn't normally go a lot higher for a bigger one. Personally I wouldn't pay more than $800 on this one (basing on the pic).
It turns out the fish for sale ain't the one on the picture !! The guy selling it was only asking 100$ for it... for an asian, that would have been quite a steal. The arowana is a normal silver.... too bad !