Make - or Buy?

Determining which systems, components or modules we should "Make" and which we should "Buy" (in extension: use from Open Source) is a challenging aspect for every IT organization. Even when there's a clear votum by management of developers in favor of one option, that vote is often formed with a myopic perspective: managers prefer to "Buy" whatever they can, whereas hardcore developers prefer to "Make" everything. Neither is wise.
But how do we discern?

There are a few key factors at play here:

Factor	Details
Availability	When there's an affordable, ready-made solution, then "Buy" to avoid reinventing the Wheel. Be sure that ready means ready and "affordable" has no strings attached.
Uniqueness	you need to "Make" anything that's unique to your business model.
Adaptability	When there's only a small need for change and customization, "Buy" is preferable. Never underestimate "a small change."
Sustainability	"Buy" only when both initial cost plus lifecycle cost are lower. Include migration and decommissioning costs.
Skill	If you need specialists that you don't and won't have, "Buy" from someone who has.
Dependency	If your business would have to shut down when the solution becomes unavailable, "Buy" puts you at your vendor's whim.
Write-off	You can "Buy" to gain speed even when all indicators favor "Make," if - and only if - you're willing to write off everything invested into the "Buy" solution.

Choose wisely - the answers are often not as obvious as they seem.