I didn't take time to read through EVERY post but I think what I have to add is what alot of others have said as well.
I've never done it...but I think you can eat for under $100 per day if you really tryed.... BUT I also think you woulnd't be able to get some of the wonderul foods that Disney has. If you'll have your money for dinning a few days before your trip I'd seriously look into buying the DDP as soon as you could. Like another poster mentioned you can add it to the reservation just a few days before and be alright (three day I believe they said). And for only $17 more each day it's WELL worth it. Not only would you be able to enjoy a number of awsome foods that with your current budget just wouldn't allow but you'd also NOT have to worry about going over your daily budget...because your food would be paid for ahead of time.
Also consider...with your currrent budget of $100 per day you'll be mostly looking at CS meals, only the bare min, and really no room to purchase any snacks ($100 for 3 people=to about $10pp/per meal)....on the DDP for $117 you'll EACH get one Table service (appitizer, entree', dessert and beverage) one Counter service (entree', dessert, and beverage) AND a snack EACH day. I'm thinking if you went to any given CS place you'll likely reach your $10pp limit by a entree and beverage alone. ...no desserts, no snacks and no appitizers.
Anywho...I think you get what I'm saying. The DDP is well worth the added $17, and is possable to get even a few days before your trip.