The rhom you pictured pumbus is a very nice specimen. Expect to pay close to $300 for a fish like that. You could get a younger smaller one at say 2" for like $25 but they grow SLOW and are VERY shy at that size. Oh and yes you can only keep one in a tank.
I suggest you really think about how much you like them. If you lvoe the fish for what it is and not the hype atached to it get it. If not I will almost garuntee you that you will be highly disappointed with a rhom. Of course unless you have a ton of money to spend on a large fish!