Given the nature of the structure and the design loads involved, the foundation design seems reasonable to me. It would be pretty short sighted to cut corners and have a major failure in the event the worst happens. A couple years ago, a nearby ice arena collapsed due to snow load. Fortunately no-one was inside at the time.
Partsguy57's comments seem to be primarily directed at mcbane's 8" thick slab, which mcbane himself has acknowledged is probably overkill. I agree with both of them. 8" is probably is overkill but it's really not much extra money on a project this size. We've all been guilty of that from time to time.
Back to the OP's original question - which I'm pretty sure he regrets asking at this point - call any ready mix suppliers within reasonable distance, ask for the sales dept. and tell them what you have for a quantity, what you want for a mix and ask for a quote. A 200 cy yard job will get their attention. I'll bet you get get a significantly better price than you would if you just called dispatch.
I place thousands of yards a year with local suppliers. It drives me a little nuts but I have to call in advance for a quote or I get charged the standard rate. Maybe its just the suppliers around here but you won't know til you ask.
