To do svt conversion not counting a new catback exhaust can easily be done for $500-$600.
As for mpg one of the biggest things in our cars is how you accelerate. If you do so aggressively your mileage will suffer alot. If you a very gradual/slow in acceleration your 25 could easily reach 30. Stop n go traffic is a tough one though. I drive pretty aggressively and average about 30-32 mpg.
Hatch Nation #9
"2 by 2 Hands of Blue."