Had some done last year...got several quotes...going rate is $200 a (cubic) yard finished (Lee County). At 4" thick, that works out to $2.47 per square foot.
20x40 should be around $1975.
How I do it....simple math:
20x40=800 sq. ft
800/3=266.67 cu.ft. required (think if you did a 12" pour at 1' thick, that is 800cuft of concrete...4" is 1/3 of 12", so divide by 3 -- divide by 2 for a 6" slab)
266.67/27=9.877 cubic yards required (there are 27 cu.ft. in a cu.yd. so divide by 27)
9.877*200=1975.30 cu.yd. times 200 (going rate per yard finished) is total cost
whatever your actual numbers are, just substitute (24x40, etc)