From the 1.6 Peugeot, a fill up is between 50-55 litres, worst i've had is 380 miles, best is 460.
In my mates D-Turbo, a fill up is between 54-58 litres (his warning light comes on much later for some reason) and he gets between 520-650.
We bought at the same time (his was much older) bought my 1.6 in May 03 for £1950, a Phase 2 Dturbo at the time would've been 2.5k. I worked out that the amount of miles to break even would be 30,000 miles. I did roughly that before switching to my current car, and i've enjoyed a quieter engine and lower service costs, but these are negated by the fact that the dturbo would be worth about 1k now, whereas my 1.6 is about 800. So all in all about even.
If you really want to save money, drop the 3 door requirement and get yourself a 5 door, or even better, a 4 door saloon, they're cheap as chips, beggars can't be choosers and all that.