You should have a price in mind for the particular service that is under discussion. Different types of gigs will have different pay rates that are either set by the musicians’ union or are the going rates of the area. Many jobs pay scale and that’s it. A traveling Broadway show or an opera put on by the local company are examples. Union scale is the minimum amount that a union musician can be paid for a particular job, but there is no limit to the amount that can be paid over scale. When figuring the budget I usually start with union scale, then factor in costs for other things like travel, cartage of large instruments and percussion rental. If you have a string quartet, for example, and someone wants to hire you for a wedding reception, it won’t be so complicated. You’ll have a price, and there will be little back and forth discussion.
But money isn’t the only consideration. I have taken lots of work that didn’t pay well, on the hope that it would lead to something bigger down the road. Sometimes it does and sometimes it doesn’t.