I also had problems with 1 powerhead, there was corrosion behind one thermostat in which the block corroded through from the inside out. When they pulled the powerhead is when the exhaust corrosion reared its ugly head. My total cost with a new powerhead was about 11grand. if you are doing just the exhaust components, the parts have gone down in price. I am gussing that replacement of the exhaust components alone would cost $2500 to $3000 per engine with labor. I would check with a few yamaha mechanics in NJ to see if they have done any replacements, they can give you a better idea-good luck
