I just had a similar experience with Lexus Woodford (less the ransom!). The missus smacked the nearside door mirror and went straight to the dealer for a quote. Before she knew it they had booked it in for this morning for a full unit replacement, at the nice sum of £329!
When I got home last night, took a look: fold in works left/right/up/down all work – absolutely no mechanical problems, all it needs is a mirror and the colour-coded mounting, which was cracked. So, took it to them this morning, low and behold only the mirror and moulding is required, price now: £120 (still high in my opinion, but better that £300+).
Like most businesses, they will always try to fleece you (1) If they think you don’t know any better, or (2) If you have no option. Then there’s the ‘Prestige Added Tax’…