Again. Depends. Depends on the event and where.
Most events, I have you pay everything up front, and I show early, setup, do it, and go. Usually it's $100/hour for just music with a 3 hour minimum. I've done other deals before. I'm willing to negotiate within some reason. I'm pretty cheap as a DJ, really. Mostly, you're paying for my time (because gas and bills don't pay themselves).
Weddings, it's half up front (DBAA fee), then half at least 35 days before the date (to keep you from making last minute changes like an asshole). I don't even take your phone call for less than $1000 for a wedding. Again, I'll negotiate a little bit, but not much. Weddings are a big endeavor, because I have to talk to your wedding planner, the venue, and (sometimes) the police.