I consider paying 15-20r for a lapis block to be a reasonable price, and it Ito be pretty standard across the servers. I wouldnt buy lapis gems/blue dye at more then 2-3 rupees a piece. My experience is of a buyer only though, the perspective from a seller may be quite different.
i sell lapis blocks or lapis ore usually. Just because most players either use lapis block for decoration, and it is more of a bulk buy kind of item. it does not really MAKE the most. Because you are really selling 9 pieces of lapis for less than 2r a piece, where as you could set up just lapis lazuli for 3-4r a piece, (which is more profit) however, very few buy this item in my exp.
