Or something like this:
2 cp per chance on a D6...
6 only = 2 cp
5 and 6 = 4 cp
etc, but max is 4 5 and 6
Then multiply that by the size in inches.
Everything up to the bold highlighted bit is fine. I've faced that kind of stuff before and it isn't so powerful it wrecks the game. The type of units able to use it (robots) should keep the numbers low.
The bold highlighted bit... AC with self repair?
How much would that cost for a 4+ I'll be back roll.
It would bring it in line with the way Imperial Titans work in Epic.