Conference rooms at hotels tend to be fairly reasonable. When my father retired, we got estimates for rates from several hotels. It tends to run anywhere from $75-125 an hour for a room that will accomodate 100 comfortably.
The difference I can see between something like this and prom is that prom is a once-in-a-lifetime event (unless you happen to go to another school's prom as well), while this is clearly not. Prom generally has a live band or DJ, free beverages and occassionally food, and is usually held in a very large building. Some schools even include photos with their attendance fees. Mine was held in the Portland Art Museum and I am positive the school shelled out a large amount of money for it.
Masonic Lodges, Elk's Clubs, Hotels, Public Buildings (rec centers and such) and Community Colleges would probably offer really great, inexpensive options versus venues that are intended solely for events.
Admission-wise, I'd suggest $8-10 if the venue allows without refreshments, and $12-20 with (depending on the kinds of food and beverage offered).
You can save a lot of money by purchasing food and beverages from Costco and Winco. Winco has good veggie trays and cheese and cracker trays for a really reasonable price. Costco has cheap, tasty cake and bulk drinks.
I've been involved with a fair bit of event planning, including several budget weddings, so if you want any ideas or suggestions, I'd be happy to offer some
Also, is this a masque, where people should be wearing formal attire but come masked or otherwise incognito, or just a wear-something-pretty-and-be-yourself event?
