The answer is, not much. Let's say it cost $.25 a piece, installed, per handset in USD (no idea...I would guess this is a bit high but not too much). So, assuming around 12.5 million handsets in the next six months, that's about $3.1 million in more cost, most likely passed on to customers.
But have you considered the possibility that they didn't want to do it for a good reason? The truth is that the plaster band absorbs more shock from a side impact, whereas metal would transfer most of it. Given the size of the bezel, I bet that part of the reason was because it made the screen more survivable. So, yeah, if you drop it, the band will dent. On the other hand, there's a chance that the screen won't shatter.
Having said that, I wish Sammy would've used metal so that I didn't have to type this in forums every time someone suggests that they should have used metal and that the only reason they didn't is because they were being cheap. Who knows? Maybe that IS the reason I guess.