Keep your money in your pocket. I went with this idea 20 years ago. I ask my customers if I would be allowed to place a sign on their yard. All that I asked said yes, I kept my money in my pocket, where it belongs. Don't even consider paying your customers to place a sign on their property.
Make sure your not in violation of any and all zoning and codes for your town, city, or county, not to mention HOA or Condo.