Keeping the asssumptions simple is the key to a good model/ spreadsheet. Therefore, we siply ignore this possiblity, since it is rare in our case. However I did do a net present value calculation, and use opportunity cost (using our current mortgage rate as the alternative to plowing the money into DVC for a good conservative assumption), so not too simple.
So our breakeven ranged between 8 yrs and 12 yrs depending on other assumptions (and assuming zero resale value).
So try not to make your's too complex, and simnply ignore relatively rare circumstances. Or if you plan to rent your points and stay somewhere else for cash then see how close the values are; I suspect you'll find them coming out around the same (or if you're making money in that circumstance, you'll need to pay tax on it).
I depends on how you set up your spreadsheet, but I would take the difference between the amount of money you got from renting and the cost for accommodations for your other vacation. Then take that difference and subtract it from the annual dues, or from the monthly payments (if financed).
I didn't finance so on my spreadsheet it would show up as a negative amount one month in the row for dues. Looks a little odd but the figures balance out to a close enough approximation.