Determined by cost of print preparation plus the unit price of selected apparel, spread over the number of shirts ordered.
Job setup includes:
Mockups leading to your job approval
Preparation of art file
Preparation of print sheet
Pretreat the shirt ( If necessary)
Machine setup
Printing
Job teardown and cleanup
Apparel costs:
Each style of apparel has its own unit cost.
Starting at 1 piece, the more shirts printed,  the per unit price decreases.