Rather than looking at the obvious data, like bounce-rate and overall conversion rate, what specific data should I be looking at to help improve earnings on a site or seeing where money is being lost?
First, you need to make sure you have the conversion funnel set up properly. Enter the urls for all of the checkout steps (cart, login/register, checkout page, etc) and then look at the conversion funnel. This will help you to see if there are any bottlenecks or major problems causing a huge portion of your customers to drop-off at any point in the checkout process. Ideally a checkout should be one page, the more clicks, the higher the drop-off.
Also look at all of your individual pages, see which ones have the highest abandonment and the highest conversion. That will help you see what you are doing right/wrong.
I'd also highly recommend doing extensive A/B testing. You'd be surprised how much difference the wording of a button or even the color can make on whether or not a user clicks on it. Same goes for navigation elements, position of items on a page, promotions, etc. You can never A/B test enough!