I'd assume they sell less because of distances. Also perhaps Crete in this regard serves as the unlawful buy point, and Dagger being the lawful one. Although Artifacts themselves are technically a generic commodity (they really are just inert alien rocks), I'd have no issue with Crete selling them cheaper than Dagger as it'd encourage activity going into Gamma rather than ending at Omega-55. The Dagger sell point is more for lore/RP reasons than economy/gameplay.