Depends greatly on the percentage of the engines power the system consumes. Petrol engines, particularly small ones, seem to be more affected than diesels, as I have mentioned before. An egg cup full is maybe a bit optimistic though. What I will say, is that there was absolutely NO difference in overall fuel consumption on my 2.0 HDi Picasso when I made the Aberdeen/S Yorks journey in July with the system on, and again in November with it off. It did 54 MPG both times. However, if you're doing a lot of stop/start heavy traffic style stuff and the engine is idling with the system running, it will make considerably more difference, purely because the ECU will have to compensate for the draw of the aircon by putting in more fuel to maintain a steady idle. You'll just have to "suck it and see". I can't imagine it will make more than 1 mpg difference to your 2.0 TDCi.
2008 2.0 TDCi Zetec