Sounds like you are talking about a metal building. Are you looking at the "corrugated "half a culvert"" type of buildings, or of a standard channel steel, red "iron" and sheet metal building?
Metal buildings, that look very similar can be very different in quality and price. you have to look at the quality of the building, what will and will not come with it, and who will erect it and how good they are. Personally you need to look at buildings made by your potential suppliers to see how they are constructed. This is not something you can do on an Ebay bid and get quality and what you want.
Your concrete work will be the largest single expense outside of the building itself. Figuring a monolith pour, with reasonable footers and piers, you are looking at some 100 yards of cement, or more, at probably $100 a yard , plus forming and finishing cost, rebar, etc, probably $15,000 for the slab, possibly more (I'm figuring a minimum four inch slab, go six inch for heavy trucks and tractors and you are looking at $20K to $25K). Then there is dirt moving and digging, erection costs, options such as garage doors, insulation, and windows, all add to the cost of the building as they require additional framework, and even basic electricity, and in a building this large, a bathroom?
Actually a 6000 sq ft building probably will run around $150K with concrete, turnkey. Sounds like your quotes are high, but we don't know where you are, you might be paying a winter premium or have high costs due to location, distance, etc.
Tell us more.
Charles