I used to have halogens up - cost a bomb to run. Bought two x250w SON's and they were £80 each or so, put out plenty of light for a 20x40 and cost literally pence to run.
I think ours is free but I ride in the mornings, so never use the lights. If someone wants the lights on, they always have to switch them off straight away as they aren`t on a meter, just an on/off switch.
I use to run a yard and I would have liked to charge for lights... but decided easier just to include it in the livery fee, but use to scare people with leave the lights on and I'll get a meter!!!
If it is the same people riding every night can't you come to an agreement between yourselves and split the money..like a tea club at work?