The problem is that it is not that simple. To calculate you need to know the difficulty of mining and the exchange of bitcoin and the currency you pay electricity with (dollar, euros, ...) for the next months. And if you knew that you would probably make more money speculating.

The difficulty changes (roughly) every 14 days and depends on the power of the Bitcoin network, which changes as new miners and new rigs are added. Can you tell me what difficulty is going to be in 2 months? In 4? In 6?

The exchange ration changes even more. Can you tell me what the exchange ratio is going to be in two months? In 4? In 6?

Basically you can not do the maths. You can speculate and make your guess, but at the end of the day its not guaranteed. Mining is risky. You can end up eating your hardware if the difficulty goes up a lot and the exchange does not. And the difficulty is going up a lot in the next iterations.