Assuming you have one of those rare projects that may attract free help from other companies or individuals, what costs are associated with making it Open Source?

Legal: Every line of code should be vetted for proper licensing and being free of contractual, trade secret, patent and governmental restrictions.

Refactoring: You’ll need to cleanly separate your code into open source and proprietary repositories.

Start up costs: You’ll need to jump-start the open source community. Some minimal marketing, PR, and sales costs are likely before you get to critical mass.

Documentation: It will be easier to get adopters if they don’t have to read the source tree to figure out if it might be valuable to them. Existing documentation may need to be expanded, edited, or ported to a wiki.

Automated testing: As you add more developers, your need for automated testing increases, so you may need to add or enhance tests.

Oversight: Until you can recruit trusted open source partners, all outside check-ins must be vetted for bugs, utility, and foolishness.

These costs are small, but if no one joins your community, you’ll waste your company’s money.

For more on starting and running an open source project I recommend: