Here's a Fermi question for you.
How much energy could be generated if every inmate in the USA rode an electricity generating stationary bike for 2.5 hours a day?
Well the folks over at Treehugger, inspired by a sheriff in Arizona who installed just such a bike to power the inmates' television, did the math.
363,641 kWh of electricity which at $0.12/kWh amounts to $109,092/day or nearly $40,000,0000 per year.
Beats making license plates.