You don't really, say if you are asking about session guys or hired guns for gigging. I do agree with what Shailat said.
I can add that it is common to hire a "contractor" or a musical director". This is a musician that will be working on your project (recording session, tour, etc.) and he (or she) in turn hires the other musicians. It's even fairly common for a producer to use a contractor that they trust.
The advantage is the director knows who is free to do the gig, who is dependable and who plays what styles well. The key is to find a player that you get along with and who you respect musically.
You need to determine what your budget is (this will dictate who you can afford). Some players are paid only for the gig/session, some get paid for rehearsal time in addition to the gig/session and some get paid a retainer salary to be on hand when you need them.
Obviously, the people on retainer have to give more loyalty. The guys who get paid by the gig/session may take something else (maybe even on short notice) - then they aren't there when you need them.
I have freelanced for years and I would never take an offer if I have already committed to a project (which on occasion has cost me hundreds of dollars) - but some guys will take a better offer even if they screw someone in the process - so be aware!
Certainly union scale is a place to start, but good players can get double or triple scale (not to mention double scale for playing guitar vs. banjo, etc.) However, in a dog eat dog town like Nashville there must be tons of players willing to hustle gigs on the side for less than scale.