$500 to refit a wayward octave key?? I'll have a piece of that action!!
I can't imagine even the most bonkers-mad repairer (and there's no bloody shortage of 'em) quoting that sort of price for a simple job - so I'm inclined to suspect that what you've been quoted for is 'putting the horn right'...and all that that entails.
What usually happens is this:
Punter turns up with horn. "It's not working...one of the keys doesn't go down". In their mind (quite understandably) all that needs fixing is that one key.
Repairer examines the horn.
Sure enough, there's a stuck key...but it's stuck because the whole action is rusted to hell,and just to get to the worst affected key is going to mean a very complex stripdown operation. And then there's the issue of the worn-out keywork, and the crusty pads, and the off-line bell.
Pretty soon the quote is well into the hundreds...for what, on the face of it, seems like a simple job.
Get some more quotes...but this time really pin them down as to what work is necessary.
If it turns out its just the octave key, and you're quoted more than $50...the correct response is "Yer 'avin' a larf. mate'.