Ran ours on 30% new veg oil in the summer. £1 a litre from the tesco.
Used to have a peugeot 205 that I run on new and old oil for about 3 years before the pump finally let go. That was a different animal though compared to the fiat.
Pump was only £25 to replace with a second hand unit.
Given that winter is coming up unless you are going down the route of a proper conversion I would give it a miss as veggie oil becomes really thick and gloopy when it gets cold.
The main problem now is the price of the new clean stuff.
When I started running it bout 5 years ago it was 40p per litre. Now in our local sainsburys one litre of their own brand veggie oil is £1.45
Common rail engines aren't massive fans of veggie oil but I reckon if you keep the percentage of oil to diesel fairly low you should be fine. However be sure to read up carefully about what may go wrong and the prices involved in replacing those parts.