What you have to remember is that there are several ways of looking at your problem:
Get just the cambelt done and nothing else whatsoever, you literally tell them to just replace the belt - this is one of various reasons there are alot of crappy Alfa's on the road. (if this is all yours requires however then brilliant)
Get the proper service done and pay for anything else that is spotted during that service - not cheap by any means but at the end of the day it is what Alfa Romeo recommend for the car to keep it running sweet.
Keeping a car like ours on the road and running as it did when it came out of the factory is very expensive if you don't have cash to spare - in most cases anyway.. but you would have the same problem if you owned and ran most cars in exactly the same class and you would have had to pay more for them second hand as well.
Hope you make the right choice, do some research and talk to a couple of independents and don't rule out your main dealer, some are very good and offer sound advice - you pay for that. If you suspect you are being ripped off then ask them to explain in detail why they want more cash, if you are still not happy then walk away and go somewhere else.
Anyway, I see from your profile that you are an accountant so with all your megawads of moolah I say go to a good specialist or main dealer and get them to do whatever they recommend LOL
ps: would guesstimate approx £280 for cambelt and setup at main dealer. My 72k service which included cambelt and all the other stuff came to £450 and yours will be less due to the type of engine etc.