I'd like to think that the best thing to do is get at least 3 estimates to make sure you don't have a price that is much too low with missing things, or one that will blow your budget & kill the whole project, but I never had any luck doing this myself. When I built my 24x36 2 story garage, I planned for a stick built structure on a foundation with a 5" slab.
I was building it myself but needed to sub out the foundation & floor. After calling 3 or 4 masonry contractors & not getting anyone to call back, I called others & waited. After 8 or 9 total, only one called with a $6000 estimate which was more than I had budgeted for. (This was 1997)
I called some of these guys several times & finally had 4 estimates that were all over the place & confused me. The first at $6000, the second at $8900, third at $12,000, & one at a whopping $16,000. It was an eye opener so I redesigned the building as a post & beam structure. (pole barn)
The slab with the concrete ran me $2000, but I had to build a retaining wall adjacent to it because of the slope. It was 4 feet high at it's highest, & made out of pressure treated ties. I never added up the cost of the wall, but I can guarantee it was no where near the masonry estimates & probably saved me about $5,000 on the foundation & excavation.
The only advice I can give you is to find reputable builders to give you quotes & make sure you know their work.
BTW, in 1997 the final cost of the finished shell with only electricity was about $14,500.