After selling cobble 2240:1 and expanding into the lower market by trading 27 cobble for 10 gold nuggets, I think about the upper market, with large cobble orders over 60,000.

2240 cobble = 35 stacks = 1 grass
1 shulker = 27 stacks = ~0.77 grass
35 shulkers = 945 stacks (60480 cobble) = 27 grass
35 shulkers @ 3 grass = 105 grass
Total 132 grass