I think there is a very simple lower limit you can calculate mining regardless of price of future BTC in dollars.

With a certain set of hardware figure out how many bitcoins you will be able to mine with that hardware. Since the returns diminish they will approximate a certain number of bitcoins.

Then calculate how many bitcoins you could purchase for the price of that hardware at the current price.

If you can buy more bitcoins with the money then do that instead of buying hardware. That will certainly be more profitable because this calculation does not even include the price of electricity.