A general application had a lot of interest and many developers are able to come out of the woodwork. Your advertising costs are low, possibly free if the project is posted on a developer's site like sourceforge. Example: OpenOffice / LibreOffice.
As you go more specialized, less developers are able and willing to help. The cost and effort to find these developers becomes high enough that the only merit to using Open Source is the developer's experience in the respective field and their enthusiasm for the project. Examples of this is GIMP or GNUCash.
Even more specialized, secure environment, or where the lawyers want someone to sue, open source does not work as well because of the lack of specific accountability. There are certain exceptions to this such as SE Linux, but these are few and far between.
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