Smooth stone- 864 per grass

Drogo is selling 864 stone for 1 grass. (4 grass per DC)
There is a constant supply to my trade chests filtering down from my store of 50 DCs
If the shops are empty, you just have to wait for it to filter down through the hoppers. If Drogo is AFK on the island you can just come back later. If not you will have to wait (or leave an alt) near the trade shops while the stone moves down.