Incidentally, if your discount rate is really this high (you mention 22% annual at one point), you should be borrowing as much as you can from banks (including potentially running up credit cards if you have to - many of those seem to be 20% annual) and just using your income to pay down your debt.

I'd say just use your cost of borrowing (probably 7% or so?) for the purposes of discounting your salary and things, and then decide whether you should borrow to donate or not based on whether that rate is less than the expected rate of return for charities. (This is assuming that you can get access to adequate funds at this rate - I'm not entirely sure, but it seems plausible.)