Maybe this will help..., couple years ago I did the slab in my pole shed (30'x48') and potting shed (6'x20'); I did all the prep work. I laid the 6mm poly, did the key joints, etc. I had a mason do it as a side cash job... charged $800 to cover his labor and two helpers (floating, screed work, and running the power trowel). I also provided my labor to help on the grunt work (raking out concrete). The truck showed up a 8 am, and they finished the power trowel work and left in their truck at 3 pm, and didn't fool around.
Back then, 3500 psi concrete with fiber mesh was $75 cubic yard. You have 27 cubic feet per yard, so a four inch thick slab had a concrete material cost of $ 0.92 per square foot (4" thick). Concrete prices have gone up considerably, I believe they are around $100 a yard for 3000 psi mix in my area now (adequate for a patio), a little more for 3500 psi- I'm sure a contractor gets a little better price, but not too much.
I ended up doing my job for roughly $2500 for 1560 square feet (4" slab), or $1.63 square foot. $1,500 in concrete, $800 labor, the other $200 covered the poly, key joints, and power trowel blades (had to buy a set from the rental place. The lowest quote I got at the time was $3,800, and several were around $4500. So those quotes were $2.43-$2.88 a square foot. I would expect over $3.00 a square foot now (for a 4" slab).
For me, my effort was worth the $1,300 bucks for a days work...