Paying Musicians To Record

In general, how expensive is it to pay musicians to record things? How is it done? Is it better to rent a studio, or just rent recording equipment, or what? If specific examples are needed, let's say I wanted to record a string quartet, about twenty minutes long, just as an example.

shouldn't people pay you to commission a piece? If your string quartet is good, you can try to get some people to commission it, but in the professional level...they pay you

The best value for hiring performers is to contact your local conservatory or if that doesn't exist, a good university music program. Make sure you have a clear contract with them (google for examples online). If you want a professional-sounding recording, much better to rent time in a studio (again, ask the university near you if they have any facilities open to the public) than just to rent equipment. If you just want a demo recording for promotion/competitions/etc., good musicians + any quiet space + borrowed or rented equipment will do.