I'd definately second getting a refillable mug. We LOVE these (and can't wait to get one of the new designs!). I drink mostly cherry coke with mine, and maybe a hot chocolate at night. But if you added up how much it would cost you to buy a bottle etc to drink the equivalent, you'd be shocked I think. And, drink is certainly something you don't want to skimp on in the Floridian heat
Oh, and if you want some water whilst you're in the parks...find somewhere that sells the fountain drinks, not the bottled water. You can get a cup of water for free (rather than $2.50-odd for bottled water)
Look at the menus online either at WDW Info, AllEars or even WDW official website and price up everything you're likely to eat i.e. 2 counter services a day and 1 ts etc, add tax etc. Oh, and remember that at TS meals you tip on the amount BEFORE tax is added...that will save you a little over a few meals
I think how much you're likely to spend is an individual thing. We've got the free QSDP upgraded to
DDP and are only going to have to pay for tips and any extras. If we were paying OOP we'd only likely have to pay for two counter services a day, as we don't eat breakfast much...some people have to have three meals a day etc
