Insurance for the 156 GTA is about the same as a subaru WRX 220bhp, not that bad.
The 147 GTA might be slightly higher.
or some other on-line quotes.
Motor tax is steep, Euro 1340 or so.
Petrol consumption for mixed urban driving is not too bad around 22mpg (Vs 28 mpg for my 2.0L t-spark).
I got between 30 and 36 mpg on long runs on good roads steady speed (not neccessarily slow).
Thrashing it on windy roads or driving in rush hour does kill consumption - you could get as low as 18mpg.
General running I had no problems, tyres last suprisingly well (especially PS2's). Brake discs and pads also last.
Servicing for mine was cheap enough, but second hand you have to worry about changing the cambelt and water pump so there will be one bigger service.
Also second hand you might have to replace wishbones every 4 years(not too dear), get poly bushed ones from somewhere like Autodelta if you can afford it, dearer in the short run but improve handling and wear much longer.
Overall for the type of car it is I wouldn't say it is that expensive to run.