To my mind the only thing that is 'free' is for them. You're providing free entertainment for their paying guests. It'll cost you money to get there, then there's your time. Your time [b]has[/b] to be worth money. You'll be using your equipment, amps, guitars etc.
Sorry but this thing really annoys me.
How about, 'hey we need people to come along and clean tables, wash up, mop the floors after our party, we can't pay but you'll get free accommodation, look at it as a free holiday'.
Everyone else will be getting paid, security, bar staff, cleaners, so why not you?
Tell 'em no.