Can't give an opinion on alfa aid, but I've noticed that the price of a lot of genuine parts, particularly suspension components, has increased dramatically recently. Maybe it's Fiats way of trying to make money out of Alfa? A GTV rear spring pan arm now costs well in excess of £300- WTF is that all about?!
General advice for obtaining quotes from garages, be they Alfa specialists or otherwise, is always ensure that you are being quoted like-for-like; VAT included? parts of similar quality? any likely extras included? (if you're having a lower wishbone changed for example, the tracking will need to be checked/adjusted afterwards- is this included in the price?). A better example is probably TS cambelt changes: How many of those "special" cheap cambelt change prices you see advertised include changing the variator (which almost always requires replacement)? Bear in mind that the retail price for a variator is now £158. Cheap quotes are often just a way to get you through the door so they can pull your pants down once the work's been started and you have no choice; the oldest trick in the book.