I don't think supply and demand is a thing with downloadable content like video games..
As someone who works in the Software Development industry with a minor in Economics, I felt required to respond to this statement. Supply and Demand has a HUGE impact on all software development, including (and one could argue especially) DLC and other follow on content like patches. Although it doesn't necessarily use the most common resources (which are usually Raw Materials for Supply and Profit for Demand) - there is still a cost-benefit analysis that has to be performed.
In this case
Supply is most easily equated to
Development Man Power (but also includes Marketing time/cost and other opportunity costs related to what your staff could be working on instead). Meanwhile,
Demand is still primarily equated to
how many people will buy it at a certain price point (which translates to Profit - aka how much $ will the DLC make). However, in the case of a Free Patch (which will almost certainly be included with at least 1 of the DLC's) other "value" is included in the overall analysis. For example, the amount of goodwill and word of mouth (reviews on Steam, threads on forums, etc) will lead back to better sales of past, present AND future content.
The most obvious display of this goodwill "value" side of the equation is the fact that DLC 5 and 6 will be free to DDE customers. That decision was still based on a Cost/Benefit analysis. Someone determined that the current state of the consumer landscape had a lower Demand then they desired. This was either seen as a risk to the sales of a future expansion pack, or a risk that people who normally buy their product late via a GotY edition or some other bundled product - wouldn't make that purchase. They then determined that reduced Demand would outweigh the cost (Supply of manpower) to produce the DLC and also provide it free to a certain extent of their customer base.
With follow on products, some of which you give away for free, it becomes a complex system of multiple interrelated Supply and Demand calculations. But to say that they don't apply would be erroneous.
*adjusts nerd glasses and returns to lurking*