How often does Poisoned Mantle drop and rot?
 

Just wondering how often it drops rots or how much it would cost to loot it?

Never.

Rots every time on red

I haven't seen one rot yet. It's about as common as anything else off Trak. 2 major loot items (mantle would be one of these), 1-4 teeth, possible guts, and spells. There's just too many monk alts for it be a rot piece.