Right, you could break the link by just making residuum cost 2 or 4 or 10x more than its nominal GP value equivalent, and then giving away some in treasures. You could probably also increase the yield from the Disenchant Item ritual somewhat at that point. I'd also rule that residuum doesn't work as a universal ingredient for all rituals, leave it as purely a substance used/required for items. You can basically have a 'law of conservation of residuum', the stuff simply cannot be created, though perhaps it can be destroyed... Mostly it gets lost! There could be processes for collecting environmentally dispersed residuum though, perhaps this is what dwarves do, mine it from deep underground in minute quantities.