Cheers. I stopped because I had it quite clear in my head that while I was willing to pay 200 or so for the sake of completeness, I wasn't going to pay 400.
Dawnguard was apparently 1600 originally, so I guess I haven't been diddled there. Was a bit worried because I bought it first, then noticed the Hearthfire thing and thought maybe Dawnguard was always 800. Phew!
#9260270, By *Official* XBLA thread