Josh, we are not supposed to give prices, but look at your question. It relly tells us nothing. We have no idea where you are located, or where this sidewalk is located. The cost to do flatwork is largely related to how far the concrete truck is from the pour. If the chutes on the truck can pour right into the forms the price will be relatively low. If the concrete has to be transported in wheelbarrows or Georgia buggies it is a whole different story. For all we know you are on an island with water access only. You will need to get quotes from some local masons.
The nice thing about concrete is that it is plastic. You dont have to think in squares only. A nice curved and flared walkway looks very nice IMHO. Ive poured and finished a few in my day. You just form with plywood that's kerf cut on the back.
" Most people would rather die than think, and most do " Bertrand Russell
10' x 20' is very strange measurements for a sidewalk ..... sounds more like a patio
are you asking how much the cost is for materials? after all this is a DIY website. what is the depth of the concrete? is there any reinforcement? are you doing the work or hiring the job out? if hiring it would get at least 3 estimates from local reputable contractors.
pay attention to the comments and questions made by jagans.
"You get what you pay for, and sometimes free costs more!"