A new "Greediness" modifier in the hard mode menu is added. When enabled, all relics cost money (besides all item spawns that aren't naturally generated and boss reward pedestals). Works similarly to other modifiers where you can adjust the specifics of how much relics cost (5, 10, 15, etc.)
Purchasing the relics outside of the shop could possibly give you discount and work WITH discount so the more you play it becomes easier to purchase relics? Could be a sub-modifier that goes alongside it when enabled.