Coming home last night I heard what sounded like belt squeak at low rpms, and it gets faster/louder as rpm increases.

I have checked the s/c pulley and it is on securely. the belt is not slipping either.

Now, I assume it is the tensioner pulley... but before I go spend $120 I want to hear some opinions.

 

Sounds like tensioner pulley. Mine's been doing that for a while now, you don't need to replace it unless it's failing; that squeaking is just the bearing getting dirty/lacking grease.

As a quick fix, you could try taking it off and put some grease on the bearing. That might fix it for you at a minimal cost.

 

Take the belt off and spin each pulley by hand and see if it's making any noise.