I came up with re-purposing this for our little charity haunt, but it could be useful to others:

http://www.amazon.com/Ssscat-PDT00-1.../dp/B000RIA95G

Yup, it's just a cat scare device. It has a built-in motion sensor (very sensitive), and runs on batteries. To further decrease expense, you can buy large cans of compressed air at Radio Shack for much less money than the "refills" Sscat sells. The trigger units fits just as well on it. It costs less, and contains a lot more air. I realize this solution wouldn't be great for large haunts with hoards of guests (oh how I envy you!), but for smaller/medium haunts, it is certainly the least expensive and easy way to get a motion sensor/air burst scare.