I don't think the greyscale camera is mainly a cost concern. I imagine the greyscale camera has better low light and noise performance, which is quite important for tracking.

The big difference seems to be that this headset doesn't have AR cameras at all, but reuse the mapping camera for some light passthrough duty.

The headsets that have AR cameras don't use them for tracking AFAIK. They all have monochrome cameras for that. The AR cameras are an additional cost that is only used for AR.