The difference between the two is that the former looks just at the cost of marketing, while the latter looks at the cost of marketing and, most importantly, the income which that marketing generates.
In other words, she looks at Return on Investment (ROI).
It its simplest terms, ROI is the number of dollars you earn for each dollar you spend on marketing.
If, for example, you discover that for every dollar you spend on mailing out a postcard, printing a newsletter or advertising in the local newspaper you were able to earn 2 dollars back, it would make sense to continue spending.
Indeed, it would make sense to spend even more. After all, if you spend 2 dollars, there's a good chance you'll get 4 dollars back.
So instead of thinking, how little can I spend on my marketing, think how much return on investment am I getting on my marketing? And if that return on investment is positive, then how much more can I spend on my marketing in order to earn even more income?