What your missing for calculation is the miles per gallon of each car. If the GT is automatic it gets an average of 26 miles per gallon, if it is a stick it gets an average of 27 miles per gallon. The GT-S gets an average of 23 miles per gallon with either transmission. So if I may, I'll split the difference on the GT and say that you get 3 and a half miles more per gallon at 26 and 1/2. So going 7000 miles the GT will use 264 gallons of gas.(7000 divided by 26 and 1/2 rounded) at the cost of 2.44 per gallon for a total fuel cost of $644.16. That was 264 gallons times 2.44 $/gal.
The GT-S at 23 miles per gallon will use 304 gallons of gas.(thats 7000 divided by 23 mpg = 304 rounded) 304 gallons of premium is 304 times 2.66 per gallon for a total fuel cost of $808.64.
Subtract, $808.64 minus $644.16 = $164.48 of savings.