It seems to me like you would do much better to look at each lawn, then decide what your price will be. Maybe advertise free estimates. You wouldn't get as many customers, but they will be better customers. The way you are going you'll be doing huge lawns for $20 when others are charging way more. I don't see how anyone could make money mowing any lawn for $20.
The Institute for Incomplete Studies found that 3 out of 4