welcome to the site, and good luck on your future purchase...I recently bought my 2002 GTP,(I love it!!) and did it with the absolute certainty that it had to be a GTP because I knew if I didn't get it supercharged I would regret it later...as far as having to use premium...maybe i'm the only one to notice but i find when i use premium in my regular cars i get better mileage out of them...so the difference in cost balances out with the additional miles i get...plus I believe it's better for the fuel system overall, remeber fuel injectors are quite costly to replace, laborwise anyway...and as Scotty pointed out the difference in fuel cost is quite small, so I highly recomend test driving both a GT and a GTP before you make your decision,(once you taste the boost you'll know what I'm saying) I got my GTP with 60k on it for under 10 grand, spread it over five years your payments will be under 250 a month.....as for mileage, in my GTP I average 21.5 mpg mostly city driving, as soon as i get to take a trip with it I'll know what highway does.. but not to worry if you still have to do the GT, a top swap down the road will get you where you want to be....
p.s. A buddy of mine lives in pompano beach, I was down visiting in december...man I love miami!!!!! but anyway the way they drive down there....you'll really want it super charged

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2002 Galaxy Silver 4dr. GTP...supercharger heaven.
IM ON IT!!!!!!