re: Replacing 20x20 wooden deck with concrete slab?

Summer 2020, in DFW, I had ~400 sf concrete patio poured for $2,500. 4" thick, #3 rebar on 18" centers, 8" footing around the outside edge. I removed an old wooden deck myself, and did the rough grading by hand.

Hopefully that gives you a good ballpark. I imagine they'd have charged an additional $500-$1000 for the deck removal, because that was probably the some of the hardest physical labor I've ever done on my own.

You also need to consider access to the patio area. Is it in an area where they could back up a concrete truck? If not, is there a gate wide enough to get a transport buggy (assume 5' wide clear path)? Otherwise, your cost will go up and quality will go down if they have to transport it by wheelbarrow loads.