Its about £150 if you order it with the car so I don't have a clue you may be able to get a deal if you try. And as for the sound you can tweak all the options for the kind of music you listen too and it sounds great! . Not enough to vibrate your door trimings but definatly enough to know its there and enrich the sound. It was way better than my mates GP active sport (without Sub).
P.S if you PM SchooweeNick he may be able to get you one cheap as he works in a Fiat dealer.
How do you tweak as ordering one.
Play with the Bass / Treble / EQ Settings on the Radio to suit the music you play .
So the sub comes on with the radio/CD, no separate power?
Good sound?