Boeing says that their test Frankfurt-to-Dulles in-flight Connexion broadband service tested out well: The three-month trial is over, and Boeing says that 50 to 80 passengers per flight availed themselves of the service and post-flight surveys showed that 95 percent were successful. My understanding is that they weren't charging during this test, but let's imagine they were. (Also, you couldn't use your own Wi-Fi card: only a single obscure card was approved; you had to borrow cards or use the business/first-class wired connections.)
Let's say 75 people per flight at $35 per flight and one plane can do a turnaround on that flight once per day, or two flights (once there, once back). That's $5,250 per cycle. Let's say that they get about 10 flights per week, or $52,500 dollars. 52 weeks a year is $2,730,000.
Now reality: planes are taken out of service for regular maintenance. Minor problems may make service unavailable at times. Some flights will be less full. After the initial interest, regular travelers may stop using the service on every flight. Let's say that they can eke a gross of $750,000 to $1,000,000 per year for that one plane.
Retrofitting a plane is expensive, potentially exceeding that $1,000,000. The cost is per plane not per route: you have to equip every possible plane because travelers will come to expect it.
The real money, however, may be in yield demand management. That is, if the airlines continue their insane pricing models in which, to quote Dave Barry, no two people on a plane have paid the same fare (and someday, no two people will ever pay the same fare for any flight).
If you can assure customers that they can have continuous connectivity in a flight, you may be able to charge a slightly higher price, possibly even a few percentage points, on every seat in a plane. As executives demand this service, perhaps they pay $2,500 instead of $2,000 or $1,500 for that round-trip ticket. At that price, and with that kind of variable pricing, the $35 (estimated) per flight for service is a tiny part of what the airline can yield.
They could almost certainly squeeze another $1,000,000 per year in higher fares -- at least until Boeing and its competitors unwire other airlines on competing routes.