So if the cost of 2 x Rally2 rallies equals cost of 6 x Rally3 rallies in ERC, the Rally3 rental cost is 1/3 of the rent of a Rally2 car.

That is quite a difference, and should make the intro of Rally3 cars much easier.
And if The rally3 class gets big, and gets good talented drivers, it will fill its pupose as a good gapfiller, amd introclass to 4wd!