Free ($.01) at Amazon Wireless. Free to new AT&T accounts.
However, I am still evaluating, and I don't think Android is as polished as iPhone OS 4 is. Plus, the GPS failure makes Navigation worthless, which is something I liked a lot fromt eh EVO (which I also took back, as it was a brick with terrible battery life).
though I love the hardware potential, and how froYo 2.2 runs on these phones, their poor phone calling and lack of basic features (and acceptible battery life) make me appreciate my old Nokia 5800 a lot more.
But, that's a digression.
Amazon Wireless is the cheapest place to get a Captivate (for a new account) as of this writing.