There are new yellow guppies in the trade and yes, they sell for more than others. There are 2 or 3 new types and sorry to say I can't remember the names off-hand. Pretty much a case of "What the market will bear" pricing for now.
Sorry Patrick but they are in fact guppies as well as guppy/endler cross. They all hit the trade about the same time. Since guppies are live bearers and share many traits with endlers, it is common to see a cross.