How much does an alternator and battery cost to replace on a 1999 Chevy Blazer?

Not sure if it's the alternator or battery. Wouldn't start a few times a couple days ago. Charged it over night and have been driving it fine all day today until earlier tonight, I ran a quick errand and when I pressed on the breaks all the lights on the dashboard flashed on and off frequently, my blinkers blinked really fast, and my radio kept stopping and starting. Oh and my air conditioner hasn't been getting cold at all.