What is the formula to calculate cost of debt

It can be measured in before or after tax.

Before tax:

Cost of Debt = (I + (M-NP)/n) / (M + NP) / 2

I = Dollar Return
M = Maturity Value
NP = Net Proceeds of issue
N = years

After Tax:

Cost of Debt = ( I (1-t) +(RV-SV) /n) / (RV+SV) /2

RV = Realisable value (ie they used $104 as this is the current price, not the maturity value)
SV = same as NP above

Source: AskMeHelpDesk (https://www.askmehelpdesk.com/finance/cost-debt-correct-formula-use-196863.html)