Monstrousity

At a rough estimate I'd say these prices. Golds - 600-700gp, purely for the fact that they are common from monsters killed on a regular basis, such as fire giants, and they are low experience. The price is boosted a bit because it doesn't cost many shards with golds. Greens - 900-1100gp, I think the amount of exp you get per charm is pretty bad if you're looking at how many shards you're using, with gold it's much cheaper with the shards. Not too sure on what monsters drop these commonly, I know bloodvelds do, quite a common slayer task. Crimsons - 1500-1800gp. Very common while slaying, a lot of the tasks (from duradel only) give you a lot of crimsons, (Waterfiends, Greaters, etc) These are great exp. Only thing is when you get to the higher familiars, the secondary ingredient is usually quite expensive, take rock lobsters for example, 1.3k for 500g granite. Blues - 2200-2400gp. Pretty rare charms and the exp is great, It's possible to get 750k exp an hour with all blue charms, probably even more, so that's my reasoning for the high price. I guess these would be the starting prices, and they would decrease over time.