I believe M mirrors are NLA from BMW, hardly come up for sale now.And you thought the seats were expensive......................................http://www.ebay.co.uk/itm/BMW-Z3-GE...360995?hash=item2f04955f23:g:dYoAAOSwsXFZLHw1
Standard ones go for £300 a set used and a repair kit is £200 a set so given some M tax £500 is about right unfortunately.
Nice mod it getting expensive now. I think mine wider £300 new but that was years ago.