You talking just concrete placement or are you talking excavation, compaction, set up and pour in place...? It is honestly too hard to tell given I don't know what original site conditions look like, how much dirt has to be moved, are they having to import water to assist in compaction, how much excavated fill will have to be disposed of off site, concrete costs per cy including all fees to deliver, rebar costs, pumping or no pumping, -- looks like you are going to have 2 mobilizations,,,, one for the footing and the second to pour the slab..... there are a lot of variables here.....
My gut is telling me that you should get turn key prices and see if it is within your budget. If not, I think if you try and piece this together you are going to wind up in a bad spot.... That's just my 2 cents worth...... Sometimes we have to pay to make sure it's right all the way around and have an insured contractor to fall back on if anything is not right... if you touch it the first thing anyone on the job will do is point the finger at you and attack your competence to do whatever you do.... Foundations are not something we ever want to try and cut corners or "go cheap" on as it will bite you seriously hard in the future......
There are times when I have wanted to save money and wished I would have just paid what it would take to execute the job right up front,,,, causes much less heart and headache in the future....
Pay now, or later...... That's our choices in these types of circumstances...