One option is a mouseclick software whereby you can track each click and where they are clicking or they have a heat map option. If I had 5 different plug-ins to purchase and I could track total unique visits to the site and the purchased amount of each one, you could easily come up with the answer. EX: 1,000 total unique visits, 100 purchases with an equal distribution of each plug-in means 20 were purchased of each. So 2% bought each plug-in but 10% converted total.