I used to sell similar services and now advise a company that works in the optimization space. I agree that monthly fees are ultimately the way to go and that the connection to your time spent should be loose at best. The main price driver should be to the ROI you deliver. One thing to consider, an option with a more modest fee in return for a "bonus" related to the creation of incremental revenue and profit. Most companies won't bite but it shows you believe in your services.