Many airlines are toying with the idea of getting rid of seat-back displays, how much revenue did those seat-back displays bring in the form of in-flight advertisement? I would presume for a long haul flight the revenue might be more than what it is for a mid-range flight.
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.
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.