For a share, you get dividends and it has a current selling price. This is the commonest method. But it assumes constant growth forever. The same rationale is used to find the cost of preference capital. This is important because debt is a common way of financing.
Companies can raise debt from lots of sources.
Otherwise, use before tax cost of debt. It is an IRR equation too. You have to work it out for each case. That is why there are so many examples in the book - see pages 175 to 182. There is a problem if the firm's capital structure changes.