The Cost of DLC

As a kind of follow-up to What Price Determines a Quality Game?, I saw that someone had linked to a Gamespot article about the latest Call of Duty: Black Ops downloadable content (DLC), while mentioning how much all the DLC has cost so far.

Call of Duty: Black OpsThe total cost of DLC so far is 4800 Microsoft Points (MSP) which equates to just over £40 ($65 USD), that’s the same price as you would buy the game for at recommended retail price (RRP), so the question this time is; is downloadable content, or just some DLC, overpriced or does it depend on how much included content there is?
Fallout: New Vegas box artAnother example is Fallout: New Vegas which has three DLC that cost £7.49 ($12 USD) each on Steam or 800 MSP on Xbox Live, but are they worth the price? Microsoft have a habit of selectively dropping the price of content in certain sales and have only recently started having regular price drops as part of the Deals of the Week (recently as in, this year) and due to their policy, developers are only allowed to give away one DLC.
Steam, on the other hand, is usually a little cheaper and has a lot more sales throughout the year, meaning you can usually pick up DLC at a much reduced price. For example, in the Steam Summer Sale, I managed to buy Fallout: New Vegas and two DLC for £13.48 ($22 USD), there’s no way I would be able to do that on consoles. Games on Demand/PlayStation Network full games rarely drop in price and Xbox Live content is only reduced in small doses, at least with PlayStation Network, they have a big sale once in a while which are quite decent.
Shivering Isles cover art (Windows)What makes things worse is people who want to complete the WHOLE game with EVERY achievement or trophy need to buy the expansion or DLC in order to achieve the 100% completion. Extra achievements and trophies are sometimes included with DLC and so gamerscore/trophy whores are usually the first ones to buy the DLC, regardless of how much content or ‘time’ is in it.
This is where the question, how much content should they include or how long should the downloadable content last to justify the price comes in? Shivering Isles was forever 2400 MSP until a few months ago or so when they finally reduced it to 50% off for a while and since I had the points, decided to get it as it’s quite doubtful that it will ever cost that again or for a long time. 2400 MS Points works out at £20.56 / $30 (according to XboxPointsConverter), yeah it adds around 30 hours of game time, but is it really worth that much?
Does most DLC meet expectations? It takes many hours creating extra content to expand the gameplay and it’s very appreciated most of the time, I think one of the main gripes is map packs that can cost quite a lot and for what? All you’re getting is new environments, maybe they’ll throw in a few outfits and/or weapons to sweeten the deal, but do they really expand or enhance the gameplay?