M.,
What you're getting for $750 is absurd. Maybe if it was for 4 hours or more, then I would probably consider it, but they'd also have to offer a heck of a lot more. Surely, you can find other entertainment for significantly less, and your daughter and guests would still have a great time. Remember, it's not about the amount you pay, but the experience you create for your child. I grew up in Brooklyn, and I know that DUMBO is a high-end area, so they're going to charge high-end prices. If it's in your budget and you can spare it, then you should do that. In this economy, you're better off going somewhere else, and put aside that extra money into college savings vehicles.
My daughters are 4 and 7 years old. Some things I've done for them is take them and their friends to Pizza Hut and a movie; bowling parties; Chuck E Cheese, things of that sort. I have also had entertainment at home (clown, face-painting, etc.), but nowhere near $750. Next month I plan to take my 7 year-old and a few of her friends to Build-A-Bear. They do parties there, too.
Good luck!