I don't think it's so much a question of 'how well does it sell' as more of a question of 'value for money'. One is related to Beth, the second is related to us as consumers.
As I said in a previous post, it's dependant upon the pricing model Beth etc use, and it concerns me (in relation to the entire games industry) that many dlc are comparitively overpriced in relation to the main games. I've noted that many indie or niche developed games, where the devs are more confined to the one game, have far cheaper or free dlc, and tend to roll it out a lot sooner. On this theme, Beth had already announced they were working on a TES mmo, prior to confirming release dates for Dawnguard...that indicated to me that perhaps they were refocussing their main effort and their main resources, which now makes me consider that perhaps DG took second place to other projects.
Certainly, Beth is a large company these days, but the economy and market being what it is the games companies need to return good profit, rather than proportional profit to cost...as I said previously, I wouldn't like to see a situation where dlc or add-ons are the main money spinners, because I think that would ultimately only hurt us the gamers.
Videogames is a business. Like it or not, the bigger and more established a series, the more its status is seen as 'premium' and 'triple A', more money will inevitably be charged for it. The competitors cannot get away with this as they dont have the status to back up the price tag.
This is a LONG established standard in the entertainment industry. Take Bon Jovi as an example, he has a huge status, and to see him live will cost you a couple of appendages, and maybe your soul to boot. A lesser established band? £10-20.
Like it or not, TES is now (quite rightly) a triple A series and will come with a price tag to go with that status. Ergo, my question of how well the game you mentioned sold is very much relevant. Smaller titles have to offer something to help themselves gain a foothold, and sadly being a gem of a game doesn't ensure this will happen. Take Gun as an example, great game, yet still flopped. Move onto the next generation of console and a triple A dev (rockstar) makes RDR which essentially is the next gen Gun and its a smash hit. The lesson there? Unless you have huge rep, you cant just release a great game and expect it to fly off the shelves, a new series needs shoving down peoples throats if you want to ensure its success.