Nearly a year ago, in my rant on digital downloads, I mentioned that part of my fear would be that, with the single entity in total control of the market, there would be no market pressure to decrease prices, and as such, prices would not decay over time.
It's time to throw another example onto the stack. After having borrowed Samurai Warriors 2 from FireMedic a year ago, I finally got a copy of my own (my kids still play and love this game). I managed to find a copy on eBay for a very reasonable $5.50 — a quick search just a moment ago show the prices tend to range from about $15 and up for new, sealed copies, and two "Like New" going for $17 and $30. Doesn't seem like an unreasonable price for a game released in September of 2006. Mass Effect, for reference, became a Platinum Hits release at $20 at the beginning of this year, and it was released in November of '07.
On 16 April of last year, the Live Marketplace launched the Xtreme Legends add-on. It adds a few extra characters and stories, but its price was a little steep at 2400 Microsoft Points — $30 for those playing along at home. I'm not sure how much the original game was going for at that time, but my guess is, being almost a year and a half after its release, it wasn't anywhere near a full $60.
If you've followed the link to the Xtreme Legends content above, you'll see that it is, at the time of this writing, still on Marketplace at its original $30 price tag. You can currently get a new, sealed copy of the game for half that cost. You can get new, sealed copies of better-selling, more recent games for less than that cost.
It only proves my point that downloadable content does not depreciate.
An apologist might point to the current "Deal of the Week" promotion (where they offer a piece of DLC for a deal each week this summer) as a counter-example. I say, it means nothing. For one thing, the "Deal of the Week" is for Gold members only, so it's partially subsidized by Live membership fees. For another, this price drop is temporary — in each case, the price goes back up to its original cost after the week is over. And for a third, the sale item is generally one of their more popular items, rather than a slower-selling item for which a price drop would really benefit.
I don't know if the person selling me this game bought the DLC or not, but of course it doesn't matter, because as we all know, you can't resell DLC, either.