By Olivier Cleynen (Own work) [CC BY-SA 3.0], via Wikimedia Commons
The vertical shaft shown above runs the generator, which is housed near the engine (usually just under it). See: Dave's answer
The maximum load on an IDG on a Boeing 737 is 90 kVA, or 72 kW. Those usually run at 0.3 load but let's say it's full load. (Source)
The shaft power off-take in a GE CF6 jet engine found on a Boeing 747 for example, consumes 0.125 kg of fuel for every kWh. (Source)
The shaft power factor for the CF6 and the CFM56 on the 737 are very similar. In an hour that's 9 kg of fuel, for a 6 hour flight that's 54 kg of fuel.
In cruise, that's about 0.45% of what the engine is already burning at 2,000 kg/hr per engine. As you can see, it's not much, especially that I've used an unrealistic 100% load.
Windmills in the bypass flow
If the turbines you want to add into the stream of the bypass are realized, they too will reduce the thrust of the engine as they'll take from it, add more weight and complexity, disrupt the airflow, and very possibly end up eating more fuel.