You're looking in the £300-400 region for a pattern part from one of the more reputable names. Some chime in cheaper, but considering the labour involved it'd likely turn out to be a false economy to go for sub £200 rubbish and risk having to pay the labour to replace it gain in short order.
The alternator itself is buried deep and difficult to access and a lot needs to be removed for access and to reinstate it afterwards. It's not a simple job like it would be on a hatchback on your driveway, so labour is significant.
Bank on 2 or3 hours labour, plus part, plus VAT, and you're probably still down the road for £700ish, which is a saving.
But, it's a risk too. The VW part has double the warranty of a pattern item. If the latter failed 20 months in you might find yourself wishing you'd gone to VW. In that sense you get what you pay for.
There's also the assurance that the genuine part is 100% compatible and won't cause miniscule voltage anomalies that upset CANBUS, that sort of thing.
On the other hand, you could, probably get a Lucas one fitted for a little over half the price. £500 saving is still £500.
So no right answer, pros and cons with either approach. Personally I'd go to the dealer on the basis that I'm driving the wrong vehicle if I can't stomach the bills that go with it, but the next man or woman might feel differently.