I think its pretty well established that the rear part of the roof from the B pillar back will be a glass roof standard without any option for a metal roof. The options that will exist pertain to the area where a sunroof is installed between the A and B pillars. So, whether or not you like the glass roof in back, if you want a model 3 you will want to get used to the idea.
Also, its pretty clear now that the primary reason for the glass roof is for additional head space in back, with the aesthetic aspect being a strong secondary reason. While comparing a metal roof to glass roof in a vacuum, I'm sure the glass roof is more expensive. But I don't think its appropriate to do that. Rather, if you want a metal roof, you have also be happy with less head space in back or less range because of the taller less aerodynamic vehicle. To maintain a 200+ mile range vehicle with a metal roof and a lot of head space, you would have probably needed a bigger battery, which would have probably been more expensive in the long run.