Authorize.net is not a merchant account, it's a payment gateway. You'll need both a merchant account and a gateway, and pay fees to both, to process credit cards directly on your site.
The fees merchant account providers quote you are going to be misleading. You pay a different transaction rate depending on what kind of credit card is used on your site; there can easily be 30+ different categories of cards and different rates. If someone pays you with a rewards card, that costs you more than if they pay you with a basic check card that doesn't have a rewards program.
In the end, you'll find that PayPal's flat fee is often cheaper. Not always, but often. Especially once you do enough volume to qualify for their merchant rate pricing (as low as 1.9%). It's not a bad deal, and much better than most other 3rd party processors.