Manufacturers are very good at these calculations.
One interesting example is that for the initial IFE systems that were deployed, the heat generated by the displays was significant enough that the largest cost to the installation was the required upgrade to the cabin air conditioning systems.
It will be a lot less than the percentage a radio antenna increases the fuel consumption of a car.
Modern planes like the 787 are made to a big degree from composites and the antenna can be installed inside the fuselage - leading to 0% increase in air resistance.
This is not accurate. I don't know of a single installation within a fuselage, because the composite has enough metal in or around it to cause interference.
The fuel concern is a significant one, as well as performance. Enough fuel or performance impacts and you can restrict the airports flown with these antennas. It's not as big a problem on widebodies, but it's the reason you haven't seen WiFi on small regional or the smallest narrow bodies until recently.