How much revenue do seat-back displays generate?
According to this article from September 2014:


  Thales estimates total revenue from IFE at \$17.6m a year for
  small/mid-sized carriers, up to \$397m a year for large carriers.


But that's for all services, not just advertising. The only specific mention I could find of advertising was this 2012 quote attributed to the CEO of Thompson Aerospace:


  Somewhere in the middle of the second question about advertising on
  the system he said: “… and we are generating somewhere between \$20,000
  and \$30,000 per month per plane with the IFE.”


The revenue is apparently generated in a similar way to internet advertising:


  [...] every 2 weeks the airline and the advertising service bureau receives
  a summary of the requests/viewings and the airline gets a check. At 1
  to 5 cents per click, the tally adds up as daily flights, sometimes
  short as an hour, record revenue and usage every passenger, every
  flight, every day, and every month


However, there's no information in the article about which airlines actually use that specific service, and I have no idea if those numbers are typical or not, or even exaggerated.

If you need more information than that and you can afford USD 4,650 to find out, this looks like a comprehensive report on the IFE industry.