Google sells the Nexus 4 two ways. There's an unlocked version that will work on GSM carriers like AT&T for sale through the Google Play Store starting at $299. That model is still sold out and there's no word from Google when it'll be coming back.
Google also partnered with T-Mobile to sell a subsidized model of the Nexus 4 for $199.99 with a two-year contract.
It's still unclear why Google and LG (the company that makes the hardware) are having such a tough time keeping up with Nexus 4 demand.