Ren Mien is just a made up name for line bred A. citrinellus, and/or possibly even A. citrinellus x A. labiatus. Notihing that one can't find in a million tanks across NA.
As far as the difference between Ingots & Mammons ........
http://www.happybreed.com/en/species.htm
Ingots may or may not develop a large nuchal hump, Mammons should always have a large nuchal hump. As Big One stated, $500 is pretty much entry level price, expect to pay $1,000 or more for a decent quality specimen, and a LOT more for most of the high quality examples that one sees online.