What is the cost of concrete?

A 20kg bag of concrete costs about $20 at most hardware and building supply stores.

You can buy it in much larger quantities mixed and poured for a much lower price per kilogram. You can buy either Portland cement or pre-mix concrete in bags at the hardware store. Concrete is a mixture: gravel and cement with varying sizes of gravel and proportions of elements depending on the application. If you do it yourself, you have to figure in the cost of the gravel, delivery fees, and "sweat equity" of the project. I agree that calling the guys with the spinning truck to deliver the correct mixture is a better deal.