What is a nitrous solenoid...and how expensive is it to fix or replace

 

the selonoid is the (if an NOS brand) silver cylinder looking object that the nitrous feedline runs to. it is an electronically controlled valve you could call it.

depending which one it is (NOS has super powershot, cheater, etc.) that are different sizes, they are roughly $65 for a new one, and about $15 for the rebuild kit if it is not damaged, just leaking or broken spring.