It is not only the new rims and tires that cost BTW it is the added weight of the bigger tires, the new cornering capability, loading on the suspension, the body mods to make the flexing stop and the bigger brakes that really cost.
Even if you don't do the aforementioned, Make sure you take a real good look at the rear supports mine needed to be re-welded when I bought the car. Also with wider (real wider) tires the cornering may flex the body into the deck lid. There are little rubber mods to stop the paint from chipping. Troll around on this site and other sites there is some great info on this subject.
Last note; I know on my old drag car... most tracks will not allow wheel spacers.
My avatar rear tire is 295/50 on factory rims with no flares... I do like the look of the wide body cars but when I looked at all of the costs to go really fast... My wallet (wife) cringed and said why? Maybe I will go that route in a few years once the economy gets better.
Just my two cents...
Enjoy the car! trust me, it can be the best way to spend some time. Believe it or not My cat slows me down and lets me relax while I tinker with things on my time off.