I don't know of it will vary state by state, but in Alabama at advance it's around $14(per a can) for the house brand and $18-35 for the synthetic arctic freeze brand but they come with the gauge so you can't overfill you ac.
Deb. I just bought freon a month ago. It's $5 cheaper at Walmart, at $10 a can. I have a gauge, which I bought separately, so I don't overfill it. You don't need a pro for this simple chore, but you need a gauge. They are $20, but you will have it forever.