My approach would be this:Now my further thoughts : listing them at Google Maps for free and taking maybe 10 percent of their improvement.
But I don t see any possibility to realize that, because how could I determine their improvement? They never will show me the numbers of the years.
setup call forwarding from the google maps -> to your virtual phone -> to them
list yourself or your company in the listings
then look for how many times your virtual phone ringed.
This way you will know how many new clients (or rather leads) you brought them.
After sending them say, 10 leads, contact them and if they want still getting new clients then they'd have to pay.
That would be it.