depends.
I use a per bar rate for most commissions, for 6 bones and rhythm section my rate would be about $7/bar. you can find union documentation like this online that explains fee structures for various types of commissions:
https://www.musiciansunion.org.uk/Files ... ation-Fees
note that those are minimum rates for union work in the UK.
I use the UK union rates for estimates, and adjust rates if necessary. Some adjustments might be positive - contingency fees ("in case of"), conductor score reductions, flexible instrumentation arrangements, or anything that will require the use of another party to complete will usually result in an extra fee. "Negative"
adjustments bring the price down - if I'm doing something for an educational institution, a long-term loyal client, charity/non-profit/prison/community organizations or working with one of my musical heroes - these are some examples of situations where I'll usually discount my fee.
If I'm selling a composition that's in my book, something I just did for one of my groups and requires no extra re-orchestration or arranging, my fees are substantially lower.
If you're doing it for a publisher or re-seller, there are different fee structures. Usually it's a combination of a per bar (or per hour) rate and possibly a commission if you're doing original work (arrangements or compositions.)
Occasionally I'll work out a flat rate fee for a commission - particularly for things that involve writing a bunch of material.
I don't do hourly for most projects, some people do this, and it works for them. Some projects where it makes sense to do an hourly rate are clean up jobs, re-orchestrations, MIDI extraction/clean up, and basic data entry (putting an older score into a notation program.)