I shudder to think what a complete unit would cost over the parts counter at a dealer. As Mick says above, you can get the separate bits you need to repair, if that helps.
Just had a quick flick through sold items on eBay, sold prices for mk3 wing mirrors seem to be £120-£150, so it's in the range for secondhand. No orange ones sold though, so they're probably rarer than some colours.
You could have luck with one of the 'parts finder' type websites, where you submit a request for the part you want and dismantlers make offers, but eBay is often cheaper for run-of-the-mill stuff.