[big-boss]

The reason for the different pricing is typically the contractors them selves and usually directly dependeent on the quality. You need to be more specific on what you want then you will get closer pricing.
I would recomend: 4in thinck 3,000psi concrete slab w/ hard trowel finish, 2in sand subgrade over 6in compacted native soil, No. 4 rebar at 18in on center at mid slab, 12 in deep min. edge footing and contraction joints no more than 20ft each way. Contraction joints can be either soft-cut methode or construction joints-contractors option.
Tell this to the contractors and not only will he think you know what you want but you will get a very nice quality job and not go broke doing it. After placement keep it wet for about a week and you will be very happy.
Good luck.