Cost of using ASP.NET

Often when they are being compared to open source languages someone will mention that one factor in favor of open source is that it's free (to an extent). My question is, when does ASP.NET actually cost money to use in terms of using the proprietary technology?

Usually when people refer to "cost" in the way you described, they're implicitly referring to TCO, or total cost of ownership. The cost is not an explicit cost in that you've paid for something directly, but rather the long-term price of using something over its lifetime.

For example, even if a particular proprietary technology is free, it may be more difficult to hire and find people who know about it to work on your project. Consequently, if it is less popular than some open-source equivalent, you may wind up paying more for the same amount of labor because appropriately talented staff will be harder to find and in higher demand.

Conversely, if an open-source product is free but has low mindshare or performs poorly, it may well be worth it to pay for an expensive, closed-source proprietary solution rather than having to learn the idiosyncracies of the open-source version.

Naturally, there is some controversy surrounding just how to measure TCO, with both camps having some valid points.