Your equation/formula doesn't provide nearly enough detail to formulate the cost part of the equation. No one's "electricity bill" is devoted solely to mining. SO you can't use that input as an accurate estimate of your cost.

What you're completely excluding is how to calculate energy usage per hour or day based on the power of the miner one is using.

Additionally, to improve on your formula you should figure out how to get predictive with the income. So based on the capability of the mining machine and the difficulty of the transactions for a particular currency you should be able to calculate potential income (or coins mined) per day, which would feed the income part of your equation and allow people that are interested in mining to assess which machine gets them the best value for the dollar (or bitcoin).