My metric was, until recently, $1 per hour per person... as I got about an hour of enjoyment for $1 in the arcade playing the few games I was good at. Hang-on and Robotron 2084. Mind you, almost half of that time was standing in line, but watching others lose was fun, too. On Hang on with the bike as controller, I usually extended twice. which was thus about 7 minutes of play for a quarter at the time. Robotron was usually level 3, so about 5 minutes of levels, plus half a minute of interlevel animation.
most games I didn't make level 2 in... Defender, Stargate, Star Wars, Star Trek... I kept playing, but spent about $3 per hour of play...
The inflation calculator says about $2.8 current dollars, so that would be about 0:21:00 per $1 which Robotron definitely still would get if I could find a working arcade nearby.
For multi-player, I count each person's time in hours. So... my $85 on Daggerheart is in need of 85 hours total per participant... 6 at 8 hours so far, and 3 at 14 hours so far...48+42= 90 hours, and given the $2.8 per hour... it paid off before last week. Hell, it paid off this week at $1 per person•hour this week