Premium gas...$3.50ish difference a tank. Fill up every other week. $91 a year. Not really a whole lot. Decreased mpg on the other hand depends on how you drive. Fully bolted/tuned and I still average about 30mpg.
Insurance...not as expensive as you would think (some insurance companies don't even differentiate the Si from the other models):
Compare insurance premiums for 2012 Honda Civic models
Premiums below are a nationwide average of rates from six major car insurance providers for a 40-year-old single man, no accidents or violations, commuting 12 miles each way to work, $500 deductible for
collision and
comprehensive coverages, 100/300/100 liability coverages ($100,000 in
bodily injury liability coverage per person, $300,000 total per accident, and $100,000 in
property damage liability coverage).
Model Style Cylinders Expected premium
Civic DX 4 Door Sedan 4 $1,281
Civic LX 4 Door Sedan 4 $1,322
Civic EX 4 Door Sedan 4 $1,363
Civic HF 4 Door Sedan 4 $1,364
Civic 4 Door Sedan 4 $1,372
Civic 4 Door Sedan Hybrid 4 $1,372
Civic LX 2 Door Coupe 4 $1,444
Civic EX 2 Door Coupe 4 $1,482
Civic SI 2 Door Coupe 4 $1,509
Higher payments are a trade-off for the money dumped into an LX/EX to make it "better." Or you could just put a bigger down payment down using the money that would have been spent on mods and get the payment closer to what an LE/EX would be.
Manual transmission...is better if you are really into performance cars.