It is worth $0, all of its drops are worth 0$, it has no McMMO drops. All it is good for is causing lag while falling. Replace all gravel with dirt or air or whatever.
This is a very impossible task, replacing all gravel in all of the maps is crazy, there are at least 2 veins per chunk with each vein having roughly 20 blocks in them. There are 3515625 chunks in the main world alone, this is roughly 140625000 blocks of gravel, replacing it all would crash the server, or even corrupt the map in bad cases.
