Customizer → Designs → click a design → Pricing tab.
A design price is a per-unit surcharge added when the customer selects this design. 0 means the design is free. The storefront adds it to the product tier price and shows the total next to the call-to-action; the quote stores the breakdown line Design: name — amount.
Can a design have quantity tiers like products? No. Design pricing is a flat per-unit amount. Put volume discounts on the product tiers.
Can I charge differently per product for the same design? Not directly. Duplicate the design (one per price) and use Include rules on the Products tab.