It’s more of an awareness play with influencers than getting easily measurable ROI like in direct response marketing. I’ve used influencers on campaigns after getting audience profiles to align demographics and interests, so I know I’m at least hitting the target audience. One also has to look at engagement to see if they have real or fake followers.
I wrote an article that is related to this in which I calculate CPM’s of influencers compared to traditional TV spots or preroll. It’s a very critical take on the influencer industry as a whole: https://medium.com/@drewminh/paid-new-world-95eb91fdbee9?source=linkShare-856efdee4fa1-1534281586