I know in the case of Moen and maybe others, they have a lifetime guarantee.
What you need to do is: turn off the water first, of course, then remove the handle for the tap you suspect is causing the leak, and carefully pull the leaky cartridge out of its well (you may need to screw in that screw for the handle first, and pull straight up with a pair of heavy pliers...there's actually a special tool for this).
Then you take this old cartridge to your Moen plumbing dealer, and ask for a replacement (not off the shelf)...they have replacement cartridges they will give you for free.
This way, you never have to replace the metal parts, as they're very well made, and will last quite a long time if installed properly. You just get a free replacement cartridge as often as necessary.