Determining ROI

Had to post somewhere, but for GPU as well.
We’re mining crypto, so the value of a steady hashrate varies with the value of the coin being mined.
If I paid for my miner in Bitcoin and getting paid in Bitcoin, that’s fairly easily to know when you hit ROI, except that a Bitcoin drop in value can temporarily put you underwater.
But if you paid in one coin and mining (and not exchanging as you mine) or paid in fiat currency, how do you judge ROI?
Example. I paid in US$, but am paid to mine in Bitcoin. The only thing I can think of is either convert the purchase price to Bitcoin or my mining gains to US$, in order to compare apples to apples.
Anyone doing this differently?