My Oldest DS did dress up last year but it was with found items. He wasn't going to dress up, wasn't going to go TOT but when he saw the other kids going he got excited and went TOT and had a blast, but I agree I'd never buy a $100 costume thats only going to get worn once. Plus new boots at WDW 
And I totally agree you can't give in all the time.
I agree with PPs have her buy them or pay half if she really wants them.
I would point out that one way to buy this costume and make it more budget is to re-sell the items when she's done with them.

And I totally agree you can't give in all the time.
I agree with PPs have her buy them or pay half if she really wants them.
I would point out that one way to buy this costume and make it more budget is to re-sell the items when she's done with them.