^Still, it is an off-element ability and creatures with an off-element ability pay for it with a stats reduction/cost increase.
From where do you get this? Pegasus doesn't pay for it, Devourer doesn't pay for it, Graviton Fire Eater doesn't pay for it....I just have no idea where you got this idea or why you think creatures with an off-element ability have lesser stats or higher cost.
They play for it via a small and sometimes unnoticed way. Pegasus pays for it via -1 hp. Devourer doesn't pays for burrow, because it is too situational (furthermore, I consdier Devourer slighly broken for that reason). Parasite has very high cost (2
) for its stats (1|1), because poisoning is an off-element ability. Anubis pays for its
immortality through high playing cost (8
), which often seems too big even for Anubis great stats (5|8). Graboid also doesn't seems to pay for it, but I consider Graboid broken.
creatures doesn't seem to pay for it (Mind Flayer & Toadfish are just ok, even as vanilla creatures) but this seems to be one of Water's special traits (going well with foreign elelents). That's my humble opinion, based on my ETG experience...