Cost of Debt – Lending/financial experts

Looking to learn about the cost of debt financing from lending/financial experts. In particular, what are the formula(s) for calculating cost of debt (accounting for taxes and interest). And how do you justify as a business owner taking on debt–in other words, how do you figure out how much growth or revenue the debt should create in order for the loan to make sense for your business? Requirements: Must be a lending/financial expert