Skipping testing on 15% of all devices to save $600? Sounds like a poor business decision.
We are a very small company, and have always had far more Firefox than Safari users. And though they get by via dominance, IE style bundling of the browser to the OS is toxic, so good riddance.
15% of devices is not 15% of users. From my own experience having a web app that is 99% desktop windows users, why would I care about safari?