RMT aren't responsible for everything. :p Demon horns are now obtainable from chocobo digging and mining in Mount Zhayolm, which the only place you can get khroma ore for making Mezraqs and salvage body gear. That's why the supply of demon horns has overtaken the demand and the price has dropped.
I have found going to the home point in Castle Zvahl Keep [S] and making your rounds on the top floor, is the best place to farm these if you're leveling bonecraft. Gear up your THF! I was able to farm 8 stacks in just a few hours with TH8.
