Whats about the going rate to put new piston and rings in a 400ex. Its puffin some smoke and and needs rings. Whats a fair price do you think?

 

I work in the service dept at a Honda/Yamaha/Polaris dealership and my best guess would be 1.5 to 2 hours labor plus parts and shop supplies.
I don't know what the going rate is on the parts from my shop but here is a price list from OEM parts finder.
Piston=$58.93 STD $89.56 overbore.
ring set=$35.93
circlips(2)=$1.04
gaskets=roughly $40.00
eng oil=$3.95 a quart

Labor rate at my shop is $65/hr.

Puffing smoke could also be valve guide seals. You should have a leak-down test done to be sure.