Cost Variance
Rule: Give a little, but not too much.
Matthew is drinking beers at $5 each. Mark is drinking rums at $7 each, and Eve is drinking some exotic crap with eight types of fruit and very little alcohol at $11 a pop. The law of cost variance states that a difference of 50% above the cost of the cheapest drink will be tolerated. So Matthew and Mark are permitted to be in the same round. Eve, in the above example, is outside of the toleration zone and should not be included in the round. (Note: Eve is a woman, see Women in the Round for complications.)