Armatus get much larger although never as large as their true wild potential. There are many myths surrounding both species as to why many of them do not do well in captivity, however both species have and are being kept successfully in the hobby for several years anyways. I'm yet to read about any Payara lasting 7-10 years or reaching lengths of 30 or more inches. All 3 Hydrolycus species do have differing characteristics, very distiguishing when they are larger however juvi's can be down right difficult to the untrained eye. Scombs do not have a curved underside like Armatus and Tats, their tailfins are also noticably smaller, they won't have a piont in the middle of the tail fin and the top of the dorsal will not curve to a piont either. I can only speculate as to price differences - I've seen Rhaphs for $20 - $80, Scombs from $20-$100, Tats from $30 -$200 but never an Armatus for less than $100+ and most over $200 lately. More than likely the other species are easier to come by on collection trips but once again it's only a guess.