Probably not the pulley but the belt.
You can get belts from Autozone cheaply (30 bucks or less). If you feel like installing it yourself, it's not too bad. Otherwise an install at Ford shouldn't be too much money.
You can inspect your current belt, and if it's in good shape (no cracks), sometimes you can cure the squeaking and get an extra few thousand miles out of the belt by taking a mild abraisive (like fine wire wool) to it to remove the shiny 'glazing' that builds up on the belt. That's what generally causes slippage and squeaking. Better to just replace it though...
