If the widget is hosted on your server then you could bracket your plans based on the traffic. So you could say $19.99 for up to 50,000 page views a month, etc. How valuable your widget is would determine how much you can charge (be sure to factor in your bandwidth costs as well if the widget is a decent size).
One other comment, encouraging people to click on anything other than the "buy now" button on a product page isn't going to be very appealing to most e-commerce people. Returns are expensive, but the last thing people want to do is deter orders.
Thanks yeah, I'll have to factor in all those percent changes and ROI's to make sure everything is worthwhile, landing page optimization is a specialty of mine so it should actually be fun :)
One other comment, encouraging people to click on anything other than the "buy now" button on a product page isn't going to be very appealing to most e-commerce people. Returns are expensive, but the last thing people want to do is deter orders.