Downloadable Content (DLC) comes in MANY different shapes and sizes, and this is a quick look at three pieces of DLC and their perceived value.
In the earliest days of the Xbox 360, Oblivion was released by Bethesda Softworks and was an amazing game (I didn't finish it though). Bethesda then went on to release what was the first DLC for the 360, in the form of 'Horse Armor' to be used on your in game horse. It was a purely cosmetic addition and Bethesda tried an experiment, by pricing the armor at $2.50 which wasn't exactly met with universal acclaim.
Since the DLC was cosmetic only and used on something you didn't really need in the game, it was easy to dismiss it and say it was over priced
The first of two map packs for Modern Warfare 2 (MW2) was release at a higher than expected $15 price point ($10 was the expected price) and was met with whining in the press and good sales from consumers. The pack contained three new maps and two remakes from the first Modern Warfare games (which was Call of Duty 4). Many people, including myself, boiled the DLC package down to "three new maps for $15 means five bucks per map." The original $60 game shipped with 16 multiplayer maps and a solo campaign, so it's reasonable to suggest that there's a price premium on these new maps.
The reason why it sold so well is that MW2 is the best game of its type and FPS players will tend to gravitate and stay loyal to one game, and in this case, it's MW2. If you want to keep playing and not be bounced from multiplayer lobbies when new maps are chosen, you need the new maps.
Since it's a multiplayer game and people usually form clans, the effect of socially obligated gaming comes into play and the perceived value of having the new maps (or just the game in the first place) is boosted.
Worth noting at this point is the comment from "amaan4ever" saying "i feel so left out for not buying this game".
In spite of the bad press about the price, and some whining on various forums, the second map pack was released at $15 and included the same distribution of new and rehashed maps.
At $25, the Celestial Steed in World of Warcraft is the most expensive single piece of DLC I've ever bought (as an aftermarket addition). It's value is that it's an enabler. The horse can run fast and it can fly (assuming you have leveled your character and have paid for the riding skill (with in-game currency)). Every last person I know who bought the mount already had another mount that enabled the player to fly and travel at very high speeds.
The reason why the star pony sold so well is that you can use it on any character you have ever created in the game and it's very common for players to create multiple characters. Each new character you create can use this mount. You can go from a lowly level 20 nobody to an overpowered death machine with this one mount under you, and it will assume all new abilities (speeds and flying) as you acquire them in game.
Having a mount is, besides tons of gold, the greatest single enabling ability in the game. $25 buys you mobility for all characters you have or ever will have in the game. That's why a small pile of polygons sold at such an amazing rate.
DLC, when presented as a trinket, isn't very compelling, but when it's delivered as an enabler it can demand a lot of real money for something that technically doesn't exist.