Follow Your Dreams
You can estimate the ROI of marketing expenses ONLY when you succeed in getting your sales to the point of success--NOT BEFORE. I see analyses everywhere that use the words "return on investment." In the riskiest career in the world--the creative career--you can't imply the cold numeral logic used in less-risky careers.
Successful artists--writers, ballerinas, painters, singers--have virtually unlimited upsides, and are among the richest people on the planet (consider Oprah, George Clooney, Michael Chrichton, etc. They did not get that way by figuring out ratios. They got that way through sheer determination and persistence, based on a decision to never give up until their dreams come true. Most of the ones I know not only gave EVERYTHING they had to their careers, but went deeply into debt to achieve success.
So, please, if you are determined to use the beancounter's approach to writing, go write business manuals for a big corporation. I give this speech every day all day: Marketing does NOT lead directly to sales. It leads to VISIBILITY. BUT, no visibility, no sales. So focus not on R.O.I. but on provable visibility. Let your destiny take care of the rest. If you don't have the stomach for this, KEEP YOUR DAY JOB.