I was faced with this very question about 5 years ago. I drew up some plans using conventional building materials, made a materials list, and priced it at several building supply stores. There was as much as $300 diference in some of the suppliers. This would be the only accurate way to do it. In my case, it was about $2,000 cheaper to use the conventional building supplies over the metel building. I ended up building my own 24X30 garage, planning every work bench, ac outlet, compressor location, etc into the project before I started building. This was truely my dream garage and a job that used to take me 2 hours I can now do in 30 minutes because I have everthing at my fingertips.
I think you would be a lot better off installing your concrete first before the building goes up. The cost will be less because it is easier for the trucks and workers to get in there to pour and smooth the concrete. Also, I would recomend a combination of fibered concrete and wire mesh to prevent the concrete from cracking. This is something you will need right from the beginning if you want to work on your cars in the shop. The slab I had pored was $2200 but I am sure the price would be higher now.