It also greatly depends on where your users are, of course. For US, it can be even higher than 12$. For many countries, it's between 2-3$ or less (with rewarded videos for us). 35 cents seems very low though, it might be that you're showing static ads only? Also, if you're showing ads to mostly the same users over and over, the eCPMs start to go down I think.
Years ago, I played quite a bit of Land Air Sea by the way, it was amazing how well it ran on my iPad 1 for something so complex! And it had way better controls than any other RTS I played with touch. I hope you've had some success with it. I hadn't seen Machines at War 3, I'll try that as well.