Why is brimstone eater the only fodder creature with a cost? I've never thought of this even while using fire rush, because I used photons or gemfinders for immo fodder, but coming to think of it, the cost might be part of the reason for that.
EDIT: Thanks a lot Higurashi for the response to my previous question
IMHO it's because upgraded they get only one more "skill". Skill may be cost reduction or quanta production.
Photon is costless but skill-less, upped as Ray of Light it gets the quanta production.
Dragonfly produces quanta unupped but costs 1
, upped as Damselfly loses its cost.
Gnome Rider produces quanta unupped but costs 1
, upped as Gnome Gemfined it loses its cost.
Ash Eater, instead, costs 1
and has no skill, upped it either gets the quanta production or loses its cost. Apparently, theme-wisely, they chose the quanta production.