ROI comes in many varieties
In this forum, people usually talk about ROI (return on investment) and results from copy in terms of hard data on sales. This doesn't apply to all kinds of copywriting or all kinds of web sites (or other marketing materials).
For example, I am now writing copy for a new web site for an online company selling a tech service used by people who by and large are not techies and who range from small local companies and nonprofits to national or multinational organizations with deep pockets.
I sent two pages of my new copy to the CEO yesterday and he is now busy trying to get his web developer to put up the new copy ASAP, within the next couple of days. Why is this so important and urgent? He has three meetings in the coming week with really important prospective clients and, he said, the current website doesn't match what he's going to be telling them in his discussions with them, doesn't highlight the uniqueness of the company and doesn't strike the right tone.
In short, he can see really clearly that the new copy could potentially mean the difference between a yes and a no from clients who could each bring in hundreds of thousands of dollars in revenue long term.
Can he prove that? Can I? No.
Without any hard evidence, the difference is still intuitively obvious enough to him to realize that whatever he is paying me is a bargain. He knows that with the new copy, he'll be much better positioned to grow the company and doesn't need to find evidence for that.
This is what often happens with my clients. Very few of my clients do any testing of copy. It's not that they're ignorant. It's that they have their eye on the big picture, which includes some unmeasurable elements.
Marcia Yudkin