Z.

Mike Dunbar could probably give a better response but since I do some hiring of players I can offer a little insight.

For studios, it is either a per song basis or you can contract for a day or mulitple days. Most of the stuff that we do at Jay Verne's studio (where I do most of my stuff out of)we usually end up about $425-$550 per song for music tracks and $75-$125 per vocalists. I am usually the producer which means I make sure all charts are done, all pre-production is done, vocals and harmonies, everything comes in on budget. When you are part of a large group of songs, say 5-15, over one or two days, it is very easy to miss things, overdubs, certain tracks, getting players and vocalists scheduled and in and out, etc. so it helps to have someone who oversees everything.
Most players are booked in a certain block of time, say three hours, and are paid accordingly, depending on if it is a union or non-union session. Some studios charge "cartage" fee's, which are basically someone to move equipment around. Some of the higher scale players are not going to move equipment so you have to factor that in. And the higher the level room, obviously the more the cost. But some things can be negotiated. The studios are usually going to have a standard group of players they use that they have relationships with, know their playing techniques, styles, etc. It is more for effeciency than anything else.
For live, most players are happy with $50-$75 for a gig. This could be a three song writers set, or a more involved live show. We use the Nashville number system, which is a number system devised for many years where the numbers correspond to notes and chords, so if you were to transpose in a different key, the first number becomes the first number of whatever key you are in.
Sometimes, writers may utilize their co-writers to play for them if they are not as comfortable as they need to be. For instance, we often have female singers who use guitar players in return for singing some of the guitar player's female songs.

Obviously, the differing gigs can mean different money. A record label showcase can range from several hundred to several thousand dollars and the players are paid accordingly. To hire a group for a few gigs, often can be negotiated depending on how far the gig is, etc. I would expect to pay at least $125 per person if it is a road gig, or very involved. It comes down to how much work is involved. Some times musicians are also paid for rehearsals, as well as shows.
Again, there are frameworks for dealing with any situation. The union has some of those guidelines and not to slight the Union, they do good work in many areas and the president is a friend of mine, but there are many times they charge more than it is worth.
A lot of this has to do with how much work is involved in something. As you are around a town like ours, you meet people and many things can be worked out to mutual benefit of all parties. For a competant bass player and or guitar player/keyboard player to come in, read a few charts on a writers night, is one thing. To plan and rehearse a show for a publishing company, record company, etc. takes a good deal more planning, and as in everything, more money.
The good thing about it all is in most cases one person knows everyone else involved. If you wanted to book some kind of limited deal, one contact with a guitar player can probably provide all the other players. Although in those cases the leader in any session or live, is usually paid more, in some sessions double or triple. And another good thing is there are a lot of people here. So finding one person to get the ball rolling is usually pretty easy.
As far as LA, New York, Toronto, England, etc. I am sure there are exactly the same things, although from what I have been told by people who move here from those areas, they are usually VERY much more expensive. I'd like to think we do a little bit of everything here. I like to say we are Walmart. Quality products at reasonable prices.

MAB