Cost of Capital

WHen a redeemable debt is issued at less than its par value,there are two kinds of returns you are paying to the investor
1)The interest itself
2)The redemption amount which is more than than the value you issued the debenture at

In case of redemption at current market value and in the case of irredeemable debt,there is no difference between issue price and redemption amount,in the former it is the same while in the latter there is no redemption amount,hence it can b concluded that the only return you pay is the interest payments in both these scenarios hence they are treated alike!

Another tid bit,whenever the issue price is at par,the required return and coupon rate would be same,if the issue price is more than par,i.e bond issued at premium,required rate of return is less than the coupon rate and when the bond is issued at discount,the required rate of return is more than the coupon rate
Also,when an irredeemable debt is issued at par value,its required rate of return would also equal its coupon rate.Hope that helps..