Where your quantity sits against the 5/10/25/50/100/250/500/1000/2500/5000/10000 break points (just under a break is the expensive place to be)
Materials and specification of the item
Quantity and where it falls against a vendor's price breaks
Finishing, customization, or add-on services
Check before signing
Setup or one-time fees charged per unit or repeated on reorders
A specification quietly changed from what you asked for
Rush surcharges applied to a standard timeline
Line items bundled into a single total so individual prices can't be compared
Negotiation levers
Standardize the specification so vendors quote like-for-like
Consolidate volume or designs to share fixed costs
Offer schedule flexibility in exchange for a better rate
About silk screen printing - plastisol ink, 1 color pricing
Priced per unit with quantity breaks. Vendors quote steps at 5 / 10 / 25 / 50 / 100 / 250 / 500 / 1000 / 2500 / 5000 / 10000 units, and the per-unit rate drops at each break.
Why do two quotes for the same item differ so much?
Usually a specification or finishing difference, not the core work. Ask both vendors to quote the identical spec and the gap normally narrows.