JohnS used to have 5 stud Alfa wheels on his car, here's a
thread where he talks about what's involved, some quotes below...
...it is not possible to get a hub convertor because 2 lugs overlap on the same PCD and only 1 can overlap for a hub convertor. You can change to 5 stud alfa like my car is but you have to buy new hubs and discs which isn't cheap
In terms of the cost I think it was about 500 quid all-in when I did it but it is best done when your brake discs need changing anyway. The most expensive bits are the rear hubs as they are a 1 piece unit that requires a little simple machining.