Start creating unit tests for the most problematic areas (ie sections of code that often breaks and causes a lot of communication between the sales team and developers). This will cause an immediate and visible impact by the sales team and other personnel.
Then once you have credibility and they see the value, start adding less problematic areas until you start to notice that the ROI just isn't there anymore.
Sure full coverage is nice in theory, but in practice it's often not necessary. Not to mention too costly.
