You really have to base it on time. When I bid jobs I look at the total overall surface condition. For example I can clean a garage floor or concrete patio for $150.00 but that same amount of surface area in front of a Movie Theatre will up to twice as long so I might carge $300. If I get te job as a regular maint I can charge a little less because it doesn't get as bad between cleanings. A typical 10 x 10 square might take an additional 1-5 min to clean based on the amount of gum so I would charge accordingly.