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.

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.