Old - About eight years ago I bought a new 60'X 100'X24.75!, 18 gauge (very important), six foot vertical side walls (then the arch starts over the top), with one 14X14 overhead door, one side entry door, all bolts (28,000), nuts, washers, etc., one end closure, for $28,500, including freight to my farm. Unloaded pallets with my backhoe. After leveling site, covering with gravel, tamping the gravel, pouring the footings (footings need to be dead on level and plumb for this type construction so the pieces fit together without leaks), poured a 5-6 inch, fiber enhanced slab, with rebar. Used my forklift with a fabricated "stinger" mounted on the mast for lifting the individual sections and loosley bolting them in place (must keep alingment near perfect). Four of us could contruct 5-7 arches (2 feet wide - 50 arches total) per day.. Took about 2 1/2 weeks to complete the arches and one end wall.
Cost for footings and slab was $15,300 (higher now). Estimated labor costs (not including me) was about $4200 (three laborers @ $8.00 per hour for ten hours, for 17 days (not including a few cold ones at the end of each day). Tools for construction were just regular wrenches and a few air guns (harbor freight), air compressor, etc.
Have No cracks in floor with backhoe, forklift, pickups, IH-424, etc., running all over it. Have had No leaks (after constructing with all bolts remaining loose - rechecking all alignments by measuring opposing corresponding components on the sides and top center - then tightened all bolts (took four of us 1 1/2 days to tighten). Have had NO sweating or leaking.
The result was a 6,000 square foot, high clearance, clear span building with a super strong concrete slab floor for about $9.00 per square foot. Made with galvalume steel and high snow load and 120 MPH wind factors.
Would do it again in a heart beat if I needed another building. It sounds like, from your description, that the building does NOT have any vertical side walls and the arch starts at footing level. If that is the case, you will have some floor area along the walls that has a low height clearance.
If that building is complete, 18 guage (or heavier) galvalume, with bolts, nuts, washers, hardware, etc., a price of $15 - 20,000 would be a steal.
If you get it I would be most happy to furnish more points of construction to avoid problems.
bob farrell