The last time I went to WDW was 20 years ago and we have never stayed on property. I struggled with this question too while planning. What I did was:
I had a general plan of which parks we would be at on which days. Then I made a spreadsheet (Excel or similar) with the days of the week and under each day I broke it out into breakfast, lunch, dinner then across I did each person in the family. Then I went to AllEars.net and figured out where we were most likely to eat and what we would most likely order. (Who knows if we will stick with this plan or not, it was just to give me an estimate.) I entered in what each person would get and added it all up. My figures came out almost $100 below the cost of the regular dining plan. And my parents are treating us to 3 days, so the first 3 days we don't have to pay for food. That made it much much cheaper than the dining plan.
I also think it depends on if you have kids and what their ages are. If they are over 9 and picky eaters, you are paying for the adult menu. With a 5 yr old it was better for us to just do OOP. Some meals he will just share with us, some meals he will get kids menu, some meals he can get adult if he wants. I wanted that flexibility with him.
But if you haven't already, look on AllEars and check out hte menus. Get a basic idea of how much it woul dcost OOP and compare that to the plan price.