I've generally seen it around $60-70, but it looks like its only $55 at Amazon currently. The Discovery Store just blew out their HD DVD versions for $30.

The BBC version is considered the better of the two. The difference is the BBC version is voiced by David Attenborough, vs. the Discovery Channel version was voiced by Sigourney Weaver, and is 5-6 minutes shorter per episode. Also BBC is 1080p vs. 1080i for the Discovery version.

I have it on HD DVD (same encode as the Blu-Ray) and the picture on this is fantastic! Very cool series!