The foundation is the expensive part. I know that part for sure!

any idea what a 30x40 slab would cost? That is the size I eventually want to put up, but I'll probably do a steel building, like these:
http://www.americanoutbackbuildings.com/Construction.htm
Personally I do not like the way those steel arch buildings look.

Have you looked into a pole building? Around here I could get one close to that size for the same money, that would be with them erecting it.

Here is a link that might be useful: http://www.americanoutbackbuildings.com/Construction.htm

That building looks really nice! I am doing a clearspan steel building right now. Mine is 60x90x18.

You are looking at between 5 and 6 dollars per square foot for your slab. Depends on how much dirt work is needed and how thick your slab needs to be depending on a few variables. You will need to get with a good concrete guy but its gonna be close to $6 per sqft!
Good luck!

I'll be doing the concrete work myself. I poured about 5,000 yards last year. I hope to get started soon. Will update on progress. If I can figure out how to post photos I'll put up some.

I visited one of the companies listed here that caters steel buildings, i hope it's worth a try also...

Here is a link that might be useful: US Steel Buildings Info

The arch type presents some challenges to erect the first couple of arches. A high-lift forklift (or plenty of scaffolding) and plenty of help are almost mandatory to get started. Once those first two arches are solidly in place, the manpower requirement is less.
Confession: I have not built one of these-- so am not speaking from hands-on experience.