You can get a Lynx bus, Number 50 I think, that will take you from Disney to Seaworld. Then you could use the trolley buses from Sewaorld on to International Drive.

The previous poster is correct,we stayed onsite last year and visited Universal and Seaworld a number of times.You can get the Lynx 50 from the Disney Ticket and Transportation centre direct to Seaworld or for Universal get off at Seaworld and cross the road to get the I-RIDE trolley or (think its)the Lynx 8 to Universal,please be aware if you get the I-RIDE you have to get off at Wet and Wild and walk over the bridge to Universal (although the walk is not that far about 15mins we went in August with 2 young children and it was much too far for them).Also the Lynx 50 doesnot run late into the night so if you want to see the fireworks etc at Universal you cannot come back by that route,we got a very reasonably priced taxi back from Universal to our Disney hotel as the kids were extremely tired by then.

Regarding the price of food we found this really depended on what you wanted,we allowed our kids to have so called junk food alot of the time as this worked out cheaper you can get huge pizzas for approx 12 dollars and the refillable mugs are a godsend if you are staying for more than a few days.We stayed at the Port Orleans hotel and there was a shop onsite we normally bought a loaf of bread and jam and peanut butter,ham etc to make up sandwiches for lunch and again didnot find the shop to be overly expensive.

Brilliant thanks for your help

Just a tip for the lynx buses. If you are planning on using more than one lynx bus for the one trip ask for a transfer when you get on the first bus. This will allow you to get on the next bus without paying a further fare. I think it has to be used within one and a half hours but will save you a bit of money if you are travelling to I drive from wdw a few times.