It's going to depend on the exact model. If you just need the undertray it will probably be around £50-£100 (I know, it's crazy, and by the look of mine the garage have used it as a frizbee.) If you need the main side panels they might roughly be £20-£40 each. Whether the small panels I am talking about are available seperately and how much is anyone's guess. IMHO the panels are a very good idea, they keep all the water, salt and grit off your mechanicals and electrics. Look at it this way, why would Fiat bother to put them on if they weren't needed, it only costs them money and they are not a 'selling point'.
If no one can tell me if these things exist or not I'm gonna have to start looking under Stilos parked in the street! I reckon they were left off after recent work was done, the exposed panelwork is much cleaner than the rest of the underside...