I’ve seen in some practice problems (forget which ones exactly) that they’ll give you both cost of equity and cost of retained earnings, and you’re supposed to use cost of retained earnings for “cost of equity” for things such as WACC, discounting, etc. I’m pretty sure the difference is that cost of equity is the cost that it would be if issuing new equity. If so, why not use the marginal figure? What’s the reason for using cost of retained earnings instead of cost of equity?