My car has a high idle?

I have a 2004 Nissan Altima. Recently my check engine light came on, and I took it to Pep Boys to see why the light is on. It stated that I have a high idle, but Pep Boys did not have the parts to fix it. Is this an easy fix, or does something like this usually cost an arm and a leg? When I am idle, the RPM's go up and down in waves.