The Airport Extreme (or any Apple router) is a must have if you want to take advantage of the wake on demand feature of Snow Leopard. What this does is allow your Mac to go to sleep, but still offer features like file sharing, iTunes music streaming, and screen sharing on demand. This works because the router will wake the Mac back up when necessary. This includes using Back to my Mac.
That alone sold me on an Airport Extreme.
As for the price, Apple tends to set the price at what they feel is competitive when the product launched, and never bothers to adjust it afterwords. Dual band for a while was rare, and I had paid $300 for a Buffalo router that offered this, and N a while back.