That does seem very low. Where is that price from?
I'd tend to say, if you trust the person involved, go for it, I can't see you beating that price.
General price from Alfa is ~£300, and you can get a stainless steel system fitted for a similar amount.
I've used them mate. I ordered an Aerial from them last Tuesday at about 3pm in the afternoon. It arrived at my work address at 10am the next morning. I was very impressed! It all looks very official aswel, as the tape on the packaging is Alfa Romeo tape... Oooooohh!