Yes its more expensive, No there is no advantage as the HD channels are generally just the HD version of normal channels that you get with your normal subscription. If you haven't got a HD tv it is pointless.
IMO (But I do like my picture and sound)
If you buy yourself a HD TV, NOT a HD Ready one, get a FULL HD one (There is a difference). Then the viewing is like the difference between VCR & DVD, Chalk and Cheese, Vinyl and CD. Especially the wildlife and scenery programs the picture differential is simply stunning and well worth the cost.
I suppose its how often you watch TV. You will notice a difference on playing DVD's and even more so if you move up to Blu Ray. Then, we download films and tv series off the net we now only download HD.....
If you get the chance pop into Technopolis or the likes and view one in FULL HD playing something in HD...You will see the difference.
Unless of course you are like my wife who doesn't see the picture difference or the surround sound difference. Techno Heathen