Look up the guild minimums for that particular position. I'm sure they're out there for some of the positions, though maybe not PA where the average is nothing or minimum wage from what I've seen.
A word to the wise - guild/union members rarely work for the minimum rates in their general agreement. At least, in the guild and union with which I'm familiar.
Of course, in a down economy you may find that people are more willing to do so.
The bottom line is there isn't an average salary because so many start at nada and many vaterans make well into six figures. You'll have an easier time guaging average freelance rates per market. I'd not look at union rates as the norm. Here in St Louis, union shooters and editors make less than 30 bucks an hour. The normal freelance rate is twice that. It's why I'm not in a union. I can't afford it.