Its different for everyone.
The dining plan only saves you what you would spend on dining without it. You need to figure out your ideal dining plan using the menus and prices at All Ears. From there you can guesstimate your out of pocket dining costs. Then you can figure out which dining plan would work best for you. Port Orleans would give you the basic dining plan and the family suites as a value resort would give you the quick service dining plan, if Disney keeps this up. You can always upgrade your dining plan, but you need to add that to the cost of your room. You then figure out the rack rate of your room, and the rack rate of buying tickets directly from Disney. That is the cost of
free dining, and the savings is your out of pocket dining costs.
Then compare that to the rack rate of your room, minus 40% and the cost of buying tickets from a discount broker while paying for dining out of pocket. That is the cost of a 40% room discount.
Then compare that to the cost of getting 1-3 days free for the buy 4, get 3 free package offer. You can either add whatever dining plan works for you, or the out of pocket costs for dining. There is your total for that offer.
I would like to suggest looking at a Fort Wilderness Cabin. It sleeps 6 and has a *full* kitchen. You also get a TON of room. Your 8 & 5 year old could take the bunk beds while the twins share the double bed in the bedroom. Then you and your SO could take the Murphy bed (surprisingly comfortable). You could cut your room costs and your food costs. If you have a car, you even get to park right outside your door.
Also, you say youre going this September and possibly next September. If you could plan your trip a week earlier the next year, you might want to look at getting APs. If you already have tickets, just upgrade them after you use them (might get you a backdoor AP discount, depending on when and where they were purchasedjust use them once first!). Then you get discounts at certain stores, 10% off many restaurants, you can purchase the Tables in Wonderland card and get 20% off most table service restaurants (cutting your out of pocket costs further), and could even get a better room only discount and if you want to add the dining plan you could without purchasing tickets. Were saving 42.71% off our room next month.