Please don't make the same mistake made by every other SEO / Web developer. As a small company ( your target customer) why should I have a web site? Don't laugh! What extra benefits I will get from my own website which I cant get from a free face book page or LinkedIn page?
Which is easier medium to reach , attract and retain my target customers?
Stop selling tasks or services. Start selling results. Go for a revenue share from the improved sales and have a 3 to 5 yr contract, with a provision to revise the share annually.
Don't go for volume driven discount. In fact pick up very small % in the beginning and increase the revenue share year on year based on the increased sales. This removes the entry barriers and gives you better returns in long term.
The challenge is that you cant offer the services to competing businesses. That becomes your unique selling proposition. You can actually ask the business to pitch to you so that you can better understand the revenue potential and work as a partner rather than a vendor.