Why are they such different prices? The markers are just decorations but what makes the shrines worth so much? As and are sculptures just for show too?
Many of the quests will require you to either own or "place" one (or multiple) markers/sculptures/shrines of a particular element. I'd typically just buy the marker when I needed it for a quest and then sell it right back for half-price.
The shrines REALLY serve no purpose because of their inflated cost. I suppose if you were high enough level (29) to afford them all and they had a "social feature" up and running, you could use gems to buy one of each sculpture as a "look at me - I'm high level" status symbol.
But pretty much, yeah, they're useless other than to trigger quests, and the markers can do that without wasting gems.