cheaper by the dozen
1. Costing less money if purchased in large quantities (not necessarily a dozen). Avocados are quite expensive at most supermarkets, but one down the street has a good deal for a bag of 12—cheaper by the dozen, I suppose!
2. Requiring less effort or accomplished more efficiently if handled by a larger group of people. This will be cheaper by the dozen if we all help, so let's pitch in and get this cleaned up!