KN,
What I will say about studio musicians in my experience. Every "true" studio musician, I found to be the best of the best. Meaning, they could read, improv., had the best chops, regardless of genre or style. And when you say to them, no, no what I want is....they can do it. This is pretty much true of the backup musicians I have seen over the years that hire out when a big name comes to town and they are hired to play what they are told to.
My point is, IMO one has to be pretty good to bill themselves as a studio musician. With that being said, the next thing to do is call the local musicians hall, tell 'em you're going into the studio and want to know what a backup keyboardist would cost per hour then price yourself accordingly.
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jam on,
Terry
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