Skip.... You forgot the gravel, set in lifts, each lift compacted. You have set up time and go back and strip time for forms. It's a 6 inch slab so there's 50% more concrete than you calced. Not to mention rebar and labor to set that up. You also have the thickened edges if you look closely.
Plus any contractor willing to work for $20 an hour's gotta be so far out of business.... probably not licensed, bonded, insured. I would doubt his work to be worth anything too. More like at minimum 45 an hour if your lucky. I think my concrete contractor makes around 60 an hour average and I'm happy to get him. Sometimes wait up to two months for him.