If you hire someone to pour and level the 20 x 20 concrete slap it will cost between $1,000.00 to $1,500.00
If you can do it yourself then you might be looking at around $500.00 to $800.00 or even sometimes less depending on the cost of concrete and delivery of concrete in your area.
If you use concrete bags to mix your own concrete then it might be cheaper but you need to factor in the work of carrying those heavy bags and then mixing them up.
For pouring a 20 x 20 slab of concrete I would recommend having the concrete come in on a concrete truck.
Just get the concrete slab area ready and add rebar and then call the concrete company and have them deliver you a load of concrete which they will then pour onto the area where you want the slab then you rake it and smooth it out.