IDG Answers is a community of experts who are passionate about technology. Ask a question or answer one below.
I think that this is quite difficult to accurately determine. I read a study a few month ago that posited something like 60% of the mobile ad clicks that occur are actually accidental. I looked for the study, but couldn't find it again, so my number may be slightly off, but not by much if I recall correctly. This fits my own experience with ads that are faux notifications, or pop up after a short delay right where you are about to click for another non-ad related purpose. I suppose that there is still some value in simply getting people to your website, whether they intended the click or not, but I suspect it could also result in significant annoyance and be counterproductive.
If you can figure out a meaningful metric, you can track your ROI, but it is a challenge. Signups, where existing or potential customers provide contact info, are much more valuable than clicks or impressions to most businesses. But how do you weight that? It can be a bit of a black art. If you can measure your organic sales, then compare sales over a period of test advertising, that can give a clue, but there are so many variables that it can be hard for a SMB to meaningfully measure the results.