Per Game is hard to say, as we play games lasting from 10 mins upto 2.5 hours or more. All I can say is that in a 7 hour shoot I will get through between 1000 and 2000 balls depending on the day and other players (Thats 143 to 286 balls/hour or 2.4 to 4.8 balls/Min average).
I do actually believe in "1 shot 1 Kill" (actually in my case 2 ball volly's), but we appear to have a lot of Wipers at our field! I don't believe in Bonus Balling, but Wipers always get an extra volly of shots mid-wipe just to bring it home!