I am a pro orchestrator with tons of experience. I'm also something else: I'm like all good and stuff at it.
Union rate?? Hourly rate? Are you hiring a plumber or something? (And lets face it, plumbers get waaay more than music geeks).
I live in the real world. I have no friggin clue what the union rate is (and I've been a member for 30+ years). Each job is different. Each situation begins anew. Measures, schmeasures. How good is your stuff? How nice are you? WIll it be fun? Will there be any good-looking women involved in any aspect of the project?
WHo's playing it? All this stuff is what the orchestrator wants/needs to know.
The problem with "union rate" (besides being non-existent in many locals, my local doesn't cover it) is that it treats everybody the same. Umm, no. No no no. Not even close. This is a real art, you're hiring an artist, not a "one size fits all" kinda person. I'm a pro violinist, my string parts will be as good as they get. My weakness is percussion. Each of us has stuff like that. Observe.
A good orchestrator can take your "Twinkle Twinkle" song and make it sound like Mahler 9. A bad one will make it sound like "meh". And money doesn't always equal "good". My cost of living is pretty reasonable, I don't live in NYC (I did) or London etc. Besides that, some people just kinda stink. And beauty is in the eye of the beholder. You might not like my style but like Joe's style etc and the opposite.
Find an orchestrator based on listening to their orchestrations. Ask them. Go from there. Be all nice and stuff.