I think the best solution would be to nerf elite graboids only, because elite ones are both much harder to kill (owls eye, RoF) and 2 more damage for the same cost... so I would say unupped graboids are balanced, but elite ones are OP.
Look at dragons, most dragons cost increases when you upgrade them, I think the same should be with graboids.
3
1
for a 8/3 creature
3
2
for a 10/4 creature
Seems more balanced, don't you think?
And I would use graboid rush to grind AI3, rather than mono-fire or mono-life, no matter if graboids are nerfed or not, because even with a nerf they are fast enough and
mark means whenever I switch to rainbow I don't have to change mark, so eg. when I drop to less than 30 electrum after a long streak of losses against FG, I can switch to graboid rush for free to win that 30 electrum and switch back.