If you are using Google Analytics on your website, it’s also helpful to instrument outside links with campaign URLs to better track referrals and purchases.  We use these campaign URLs on all of our email campaigns as a secondary form of tracking since they allow us to know which customers made gifts after clicking on a link in the email and how much their gift was for.  We also use them for unique URLs in print pieces, by creating a short URL and redirecting it through the GA tracking code.  Use their handy URL builder to create your links: 

Then, you can go into Google Analytics and find your results under Traffic Sources => Campaigns.  In this case, we are looking at the e-commerce screen. 

Using this method will give you much more data to work with when you do your eventual ROI calculations.