Murals are expensive.
But they are also cheap. Cheap in that the size of artwork you get is much much larger than a similarly priced painting.
In general, plan to spend at least $5000 minimum and $10,000 for an average mural and up to$20,000 for a larger mural. This includes the artist fee, lifts or scaffolding, paint, and other miscellaneous materials. If you try to spend less than $5000 you aren't paying the artist, all of that money is going into everything the artist has to buy. You probably aren't going to get a very good artist at a super low price point.
It is a good idea to interview a couple of artists and get an idea of some designs for the building.
Collect ideas, think about what part of the building that needs covered. Is it in good repair? Will it need work done before the artist comes in to do the painting? Why are you doing a mural?
My very own Pinterest mural page of murals I enjoy: https://www.pinterest.com/nicolereneeryan/murals-and-public-art/
Philidelphia Mural Arts Program http://www.muralarts.org/
Golden Paint Technical info http://www.goldenpaints.com/technicalinfo_murals
Mural Routes Resources http://www.muralroutes.com/resources.htm