i got curious like you and instead of buying a second dirt miner i upgraded it, turns out the cost to upgrade it to lvl 3 is 354, assuming that buying a third miner will cost 300+ this would be where the upgrades come in
i am still figuring things out but i am starting to work this out one by one.
So we have Dirtminer A (level 1) doing 0.00004 for 50 (Correct? Cant remember price for first one, but it doesnt really matter), (level 2) 0.00008 for 250 and (level 3) 0.00016 for 354. Total of 654 for 0.00016
Or we have Dirtminer A (level 1) and Dirtminer B (level 1) doing 0.00008 for 50+152=202. For it to be a better option to upgrade Dirtminer A to level 3 then a Dirtminer C for 0.00016 have to cost 452+.
I just bought a Dirtminer B and it turns out that Dirtminer C costs 264, so getting 3 Dirtminers doing 0.00004x3 costs 466. So 188 cheaper than a level 3 miner.