Jan Böhmermann published an amazing expose on political WP editing in Germany; it gets good around 15 minutes in. In the video he exposed the workings of a paid editing farm run (by Olaf Kosinsky (WikidataCheckUser discussionarchived PR-services site), an excellent long-time editor with over 3 million edits. 

We need to distinguish paid editing from general COI editing.  Paid editing is COI editing by professionals, who have strong external incentives to persist, no leeway in the outcome they are aiming for, experience in doing this in dozens of cases, and may have colleagues who can drop in as 'uninvolved' editors to forge consensus or social proof.[1]

This is one of our great recurring challenges, siphoning off both our reputation and our community.  There are many things we can do about paid editing, starting with maintaining paid-editing metrics and a dashboard of known and estimated paid editing.  We can estimate its prevalence by the availabiity of services online[2]; and look for patterns of such editing on wiki.  Even with large error margins, this would be a step above simply waiting for outbreaks to be discovered and reacting to the visible bits of the iceberg.

What sort of metrics like this do we have already?  Who is working on such things?
Since the above video came out, de:wp started a table of WP editing services. It currently includes an initial dozen examples, with no estimate of activity (the 1 account known to be associated with each is in most cases blocked; but most have active websites soliciting work) This would be useful in all languages. 
  
SJ 

 [1] as Melmann wrote recently: "in my experience, all the most difficult edits are WP:PAID. Most non-paid COI comes from a place of desire to make things better, and often can be relatively easily guided towards a better place... [or] it is relatively easy to use existing enforcement mechanisms to to correct and ultimately control their behaviours. PR professionals, on the other hand, are subtle and sometimes downright deceptive, and it takes lots of effort to check their edits when most of the time you lack context and expertise and you really have to research in depth to see their edits for what they really are. I think that one of the fundamental mistakes of the current policy is lumping paid editors with general COI editing as paid editors are fundamentally playing on a different level in terms of PR expertise and incentives"

[2] Just searching for this online led to ads from dozens of services.  The first 10 below seem to be clones of the same service (perhaps run by the same farm)
 Elite Wiki Writers
 Wiki Curators
 Wiki Genies
 Wikipedia Legends
 Wiki Page Writing
 Wiki Page Creator
 WikiProfs
 Wiki Specialist LLC
 Wiki Writers Workshop
 Wikipedia Publisher
 Wikipedia Services
 360 Ghostwriting
 Contentfly
 Otter PR
 Premium Content Writing
 ReputationX
 Upwork