We've always offered a volume discount based on the dollar amount of the order. $200+ = 5%, $500+ = 10%, and $1,000+ = 15%. We do this because we recognize that setting up (opening a job, loading materials, outputting the job etc.) to producing a job is a separate and nearly flat cost from the time and materials consumed in producing any job. Thus, that flat set up becomes less of a factor in a larger order.
Example:
Production of 50 Labels @ $1.00 each = $50.00
Set up cost = $50.00
Total = $100.00
Cost per label = $2.00
Production of 1,000 Labels @ $1.00 each = $1,000.00
Set up cost = $50.00
Total = $1,050.00
Cost per label = $1.05
We recognize that our discount schedule does not exactly match up to the logic, but it's easier and also accomplishes the second objective of generating larger orders.