"WoW" Had no idea 525 CMI Headers were this Bad. Check out the response I got on Offshore Only. Forums/General Boating Discussion/CMI Exhaust Headers ( Warning ) 1,467 Views, & 44 Replies. Replacement Cost from Mercuey Racing now $8,800.00 + Tax per set. Cost two rebuild Two 825's Mercury Racing Engines damaged from the CMI Headers Buying used CMI Headers, is just buying someone else's problem, so they can buy new.