Wow so the trip was a lot of fun.
Ok so eating in saved us a ton of money, exactly how much though is hard to say. It was 13 of us and we had to go to the grocery store only about twice once we were there and each of us paid only 30 or so total for groceries (chicken legs, hamburgers, hot dogs, rice mixes you make on the stove, etc). We brought with us a good deal of food to last through the week too (personally I spent around $40 on groceries to bring). ~$70 total/week
The only problem we ran into was drinks. I found that we had to stop by gas stations and pick up extra 12 packs of soda, waters and juices for everyone since we kept taking them, drinking a little, then setting it down and never drinking the rest. Something that did help this was bringing along those powdered drink mixes used to make iced tea. The place we were staying at had a pitcher so just add water and a little sugar and boom, instant beverage (once it cools in the frig of course). While there I made some delicious chicken drumsticks by marinading them in italian dressing for about 24 hours then grilling them. About 10 hours before actually cooking them I sprinkled on some of the powdered italian dressing from the packets to add a little more flavor, this made the chicken 10x better. While cooking the chicken I just threw on some aluminum foil wrapped potatoes with olive oil and salt added to the skin over the grill for about an hour. Most other nights we just threw in some frozen pizza's in the oven, made some instant mashed potatoes along with those rice packets that you just need to add to some boiling water and butter, and ramen.
For breakfast: cereal, bagels, and eggs.
Lunch: Since we were on the beach most of the time during this time I just made some sandwiches. We bought lunch meats and cheese from the grocery store but my girlfriend brought along some tuna in cans and we just mixed that with some mayo and made tuna sandwiches too.
What I did that was a really good idea was that I took small ziploc sandwich bags and filled them with about 4-5 tsp. of spices I had in my house ( kosher salt, table salt, black pepper, paprika, dried basil, oregano, etc) and then put all the small bags in a large ziploc bag.
We constantly used the salt and pepper for everything and the other spices were useful here and there and since they were all together in a small bag it wasn't much of a hassle at all.
Nothing else comes to mind other than just the fact that we made it (food wise) without any problems. Bringing along just random snacks like gold fish, crackers, tons of chips, popcorn, etc) was not only good for drunk snaking but also useful on the long car ride so you don't waste money at gas stations buying snacks.