If you're knocking 'em down (or trying to knock 'em down) then you should help put 'em back up.
I practice this. Some days, I'm not there to knock 'em down, I just shoot at the ones that swing. Or at rocks. On those days, I still go out to help reset (most of the time). I figure that the more people who help, the faster it gets done. And the faster it gets done, the more time there is for shooting.
If it were me, I'd double the fee for the range. But then I'd give $5 back to the people who actually helped out. That way, everyone contributes!