Serious muralists usually will not respond to an RFP unless the amount of the commission is substantial. Most experienced muralists are happy to participate in a lower-paying project if they don't have to make a proposal in order to be selected. Proposals are a lot of work and require a big commitment of time and energy in order to do a good job. This varies greatly depending on the circumstances. As an artist, I certainly encourage you to pay as much as you can possibly afford.
Murals require a tremendous amount of time for research and design, planning, and finally painting.
Artists have to pay bills and eat just like everyone else. Small projects by local artists should be do-able for a few thousand dollars. A large, important project by a nationally known artist for a large city or major corporation could run into six figures easily. Most projects fall somewhere into the lower range of the middle, in my experience. But speed or slowness are no guarantee of either quality or efficiency.