As others have said, the question is very difficult to answer. First of all, "if you spend X on marketing..." Marketing is not one activity. Surely if you can say all my marketing dollars are spend on SEO or PPC then there is a direct correlation to sales.
I would then take a look at the history of what you have achieved in your prior campaigns and then you might be able to get a range of expected returns. Second, I would look at the competition. There might be useful information that shows the marketing expenditures vs the total sales if you can find companies in the same category. For example, if I were developing a new E-commerce shoe business, maybe I can look at Zappos' statistics or Amazon's statistics. You might have to do some digging but it might be worthwhile. However, even in these cases, you will get the AVERAGE return on marketing and not the INCREMENTAL return on a campaign. So it will depend on what you are trying to achieve.
You can potentially look at other benchmarking or statistical studies conducted by companies such as Forrester or Gartner or an independent research house. They can provide statistics like % effectiveness of a direct mail or SEO or PPC campaign. But of course you have to take it with a grain of salt as the offer, the timing and the product/service will have an impact on return.
You might be able to do a small test in geography or for part of your product line. These small tests which are relatively inexpensive and can be done quickly e.g. over two weeks, might be able to offer insight into the return on marketing investment (ROMI).
And the final issue relates to the concept of "marketing." What activities are including in the marketing program? Is it a single marketing program or campaign e.g. SEO or PPC, or is it an integrated campaign whereby you combine PR, webinars, direct mail, SEO and PPC? Therefore the first question I would have to ask relates to what is the marketing campaign? The more integrative, probably there is a bigger return.
As an angel investor, it is always great to have perfect knowledge and answers to questions such as if I give you $X how much will I earn. Unfortunately, we know it doesn't work like that. I do believe you should be able to show the investor what worked in the past in your company, what has worked on AVERAGE in other competitive companies, and set up a marketing plan that shows you understand HOW TO MARKET and make course corrections along the way.
I will leave you with this one quote from Jimmy Dean: I can't change the direction of the wind, but I can adjust my sails to always reach my destination.