Yeah, I've always noticed how you can buy Disney stuff outside of the parks for much less. Good idea! You might also want to hit up the Disney Outlet when you're down there. We've actually found merchandise that was still in the parks for much less than park prices.
Oh, and I've noticed that they sell Disney princess dresses at Target and through catalogues for much less than park prices (easy to beat $80
). And, Halloween will bring princess dresses to Party City, Walmart, Target, and other stores. You could get a dress or two during end-of-season sales, then present it to her at the park (she's bound to see a bunch of other little girls dressed as princesses, and will likely want to dress as a princess, as well). She's a little young at the moment, but you might try that idea for a later trip when she's 4-6 years old.