Needs to account for hardware depreciation too. If you buy a miner for 6M sats, and you plan to run it for 3 years, you need to add 166k sats depreciation to your monthly cost to get to realistic returns.

The main things to keep in mind:

1. Network difficulty growth rate (4.5x over the last 3 years!!!)
2. Reward halving schedule

Note: you do not need to worry about exchange rate unless you have liabilities in fiat, so ultimately, try to get power from your own renewables instead of the fiat grid, and you can mine free of slavery. sats in, sats out.