About how much would it cost to replace an alternator?

I have a 2002 Ford Taurus. One day when I was driving home the battery light came on. I thought nothing of it was going to go to my dad's to see if he thought was the matter. Well the next night I'm going home. The dash panel dies, the lights and the gauges, it sounded like I had my turn signal on when I didn't, my headlights and taillights slowly dimmed then died. I ended up having to drive home with no lights for about 2-3 miles. Do you guys think it might be something else, and if not how expensive of a fix do you think it'll be?