I had a ZS TDi before the 180...
The 180 is MUCH cheaper to run, and much more reliable
In the three years of ownership my TDi had the following (that I can remember)...
4 Head Gasket Failures
A New Engine (Diagnosed Pourous Block)
Six Boost Hoses
Front Sub frame fell off so new one of them
Top left (as looking from front when bonnet up) engine mount fell off, so new one of them
Two New Turbo's (Tied in with Gasket failures)
New Fuel Pump
SRS faults
Aircon Compressor failed, meaning replacement which consisted of removing half the engine to get to it (apparently)
clutch plates collapsed (too much torque ffrom the engine for the clutch)
That wasn't modified at all...
In comparison the 180, in 5 years, has had... errr...
Gearbox (caused by shoddy repair before I got it)
Clutch Hydrolics
Mind you, I was doing 1000 miles a week in the Diesel
As for cost of fuel for these cars... Diesel is a good 10p more expensive in some places, the TDi I got 500 miles to a tank on a good run, the 180 i can get 400 miles to a tank on a good run... So... errr.. not that much in it economy wise when driven nicely. HOWEVER, put your foot down in the 180 and you can see 200 miles per tank and less, put your foot down in the TDi and your still seeing high 400's