I don't know about a study but from personal experience, over the last 4 years, we have had growth of about 90%, 60%, 40% and right now we are tracking at 45%. The big jump in the first year had to do with some changes we made such as leaving PayPal and going with a regular merchant account.

We have spent virtually nothing on marketing, $25 - 30 a couple of years ago on google adwords and that was it. We have had terrific luck and are ranked #1 or 2 in most of our key words, even over the manuafacturers in most cases.

Hope this helps.

Mike
www.CubanFoodGuy.com