N.W.
I would say that getting her a gift card would negate what she did for you. She was trying to save you money, if you spend the money on thanking her...then did you really need to save money? With DH out of work, I'd just take the party as a gift without any thought of money in return.
I would have your daughter write a very nice thank you card, and maybe include some fun details about the party and why she loved it so much. Depending on how old she is, she could draw picture or you could send a photo from the party.
If you really feel you need to give her something, I suggest a gift, not something with money value attached like a gift card. Bring in donuts, bake a cake, buy her a plant or flowers, packets of coffee or tea etc. Or just keep your eye open for an opportunity where you could return the favor with a favor of your own!