There are a couple of different ways you could execute on this.
With one, you could create separate google analytics accounts for each domain. Then in Magento Admin > System > Configuration > Google API, you would select the website or store scope for each of your sites and enter your separate Account Number for each of them.
Another thing you could try would be to set up ‘goals’ in google analytics for conversions that take place on each of the various URLs. Then you could run e-commerce/transactional reports in GA against each of those goals to get the information that you need.
Or, if you want to get really fancy, you can modify the Magento Google Analytics code on the order success page to pass through different ‘Affiliation or Store Name’ values for each site’s transactions. Doesn’t seem necessary though when you can do one of the above.