To be honest, chances are that you will not be able to calculate the cost of debt using the balance sheet only.
The cost of debt can include redeemable and irredeemable bonds as well as loans.
The cost of the loan might be given in the notes but it might be not be included (for example the interest rate for the bank loans will probably not be given as might not be fixed)
The cost of Irredeemable bonds is the coupon rate (after deducting taxes) divided by the market price. The balance sheet will definitely not include the market price but you can get that online.
The cost of the redeemable bonds is more complicated and you can use the internal rate of return to get it. You use the following cash flows:
1. The initial amount paid to buy the bond (the market price ) as an outflow
2. The annual or semi annual or whatever coupon payments are made (after deducting tax) as inflows
3. The final redemption price which is usually the face value
You will then need two interest rates which will give you one negative NPV and one positive NPV.
The weighted average of the above is the cost of debt.
From the above, you can understand that calculating the cost of debt from the balance sheet might no be feasible unless the company has simple debts (one fixed rate debt ) etc.