I don't think you can look only just at finances.
For me there are two non $$$ considerations that I look at. 1) having to book all your ADRs 180 days in advance. Which simply means that you had better be prepared to plan out all your park days 6 months in advance and 2) just the amount of time it takes to get a family ready and transported within the day to your reservation. Time, in many ways, can be a drawback to the
DDP (free or otherwise).
Financially I like to think that if you are going Deluxe resort, that the RO discount is preferable to free DDP. If you are doing a moderate, which for us, as a family of 5, the free DDP way exceeds the RO discounts.
Also, bear in mind that if you are paying oop for your food, you'd probably spend a lot less than if you paid for the DDP. I calculated that the DDP would probably cost us about $2000 for our (hopeful) upcoming stay. I think we can get by for at least $500 less than that, so when comparing RO vs free DDP, that has to be taken into consideration. I also have to remember that every time we leave a TS dining, that I'm dropping no less than a $20 tip on the table.
I also would be surprised that you couldn't do better than a 20% discount at the CR. Even if you had to pay for one AP, you could get an AP discount that would probably get it up to 25-30 or even 35% off. That would also allow you to buy the TIW card, which might offer you better value than free DDP.
Bottom line, its just not as simple as saying "here is what the DDP would have cost me and here is what my room discount would have been". If you are staying deluxe, I would certainly consider more closely the RO discounts.
Good luck.