Replaced mine the other month after it had been clipped and shattered, the glass from main dealers cost just shy of £60 with VAT. Don't forget it has a heating element built into it so, it's not just a plain glass mirror which many cheap replacements are. You may be able to get the correct part on fleabay for much less though.
It is held on by a circular plastic retaining ring with clips situated in the middle of the glass, about the size of the base of a coffee mug, and there is a small 2 wire electrical connector.
Mine was already broken and just fell off the retaining ring but, I have heard of people struggling to remove them so it may be that the easiest way to replace and remove an unbroken one might be to break it!