Since you are going with friends, your transportation may be restricted. If I were in your shoes, I would plan on going where my friends went, and simply budget for the one day at Universal Studios. I may be in the minority, but I think $600 is enough. I have taken 4 kids to Disney(in a studio plus at the BWV) for 5 days with $1000, and brought home half of that. We had AP's, a few Q&C vouchers, a few VKL vouchers(we used at Rainforest Cafe), and I packed enough breakfast food for each morning, and a few quick items for dinners in the room(basically a box of easy mac, sandwich bread/pitas, deli meats and cheese), and a few snack-type items. We drove, and had enough room to pack lots of bottled water and soda, but you could easily purchase these items once you arrive for about the same price as home. I went to the Winn-Dixie near Disney and picked up fresh fruit, milk, and juice(I spent @$20), and picked up a pizza at the Pizza Hut nearby for our first-night dinner($20), and a big salad from Olive Garden($14---and it lasted through 2 meals). We made lunch our big meal some days: we ate at Tony's in MK, Teppanyaki in Epcot, and had dinner at Rose and Crown in Epcot one evening, so I would not say we ate at budget restaurants. We took advantage of the meal certificates at AK($12 each for me and oldest DD, and $6 each for the 3 younger kids): for @$50 we had lunch, a drink, and a snack later. For you, it would be less than $20. We packed a small, collapsible rolling cooler for the waterpark with sandwich fixings, fruit, and water bottles/sodas. When we left Blizzard Beach, we went to Rainforest Cafe for an early dinner(4-ish)using one adult and 2 children's VKL vouchers, but we could have easily gone to Peppermarket Grill and used our Q&C vouchers instead. We used our Q&C vouchers at Tangerine Cafe for lunch and Beaches and Cream for dinner: I used 2 vouchers for 3 of us at Tangierine Cafe, and added 2 kids meals(hamburgers--$3.50 each w/drink), and I used 3 vouchers at Beaches and Cream and added a bowl of chili and a coke, and we waddled out! We had Mickey ice cream bars a few times, but we always took our own water bottles into the parks(except for AK). The kids had their own Disney Dollars for souvenirs they earned before we left... I suggest you do this for your son for 2 reasons; kids tend to spend more carefully when it is their own money, and it keeps you from spending your limited money. We allow our kids to spend their money on anything they want while at WDW, but when the money is gone, they get no more(and I am much better about doing this than my husband). Our single biggest expense was gas($115...we own a gas-hog), the meals without vouchers totaled less than $250, and the groceries for the room and a few incidentals(mainly Mickey ice cream bars)was another $50, plus pizza and salad the first night($34). We ate a fast-food lunch on the way down($30), and Cracker Barrel on the way home($40). Our room(DVC), like yours, was "paid for" before we left, we had tickets like you, and a few meal vouchers like you. Even factoring in the day at Universal($90-100 total) and the $60 room, I think you will be fine with the remaining $450, considering you will have a kitchen. All I had was the "kitchenette", and we managed to have breakfast in the room each day.
