The estimates vary so much because they depend on quite a few factors. One important factor is soil condition. If the soil is bad enough, a suspended foundation may be required, which will be more expensive than a typical slab on grade. Another factor is how much site work will be required. If you go with a slab on grade, the site may require 5 feet of structural backfill, or it may require 6 inches, leading to obviously different prices. And one more factor is what goes inside the building itself. If you have a suspended foundation, using CMU walls everywhere will require more reinforcing steel, and maybe deeper beams than if you go with metal studs inside, or just a big empty warehouse. I apologize if you already know all this, but since you gave a fairly general question, a general answer was in order.
