Why don't you google it? A re-tune is generally around $650 plus, but have no idea why you bought a 2.0L Turbo car if your worried about fuel consumption.
For one, an auto car is less fuel efficient than a manual, and even being a GTA, it is still classed as a performance vehicle, so fuel is always going to get drunk faster than if you'd got the 1.8 naturally aspirated equivilant.
All you should do is make sure your GTA is running as sweet as it can be, by going through he normal things like, cleaned injectors, clean filter, less restictive exhaust, wheel bearings in good cond, wheel alignment set up for normal driving, service the transmission with filter and oil change. Then at that point you can elect to get the economy tune done, although if you don't get strange looks when asking for this, i'd be surprised, but even a good tune will make your engine more efficient, which will need even less throttle to do what you want it to do.