It'll probably cost quite a bit more than you think. For example, in flight entertainment systems are way more expensive than the cost of the touchscreen monitors because they require a power subsystem that draws from the plane's generators but is completely isolated from the rest of the avionics for safety reasons. This requires literally miles of extra wiring for a jumbojet which is very labor intensive. For something like the Boeing Dreamliner, which has about 60 miles of wiring, that's a significant expense.

That's not including the extra cost of maintenance for yser facing electronics that the trains have never had before. That said, airports like SFO and LAX have had charging ports and plugs for a while so there's data to estimate from.