Looking for a little more HP on a budget.
I'm not disagreeing with anyone here. Depending on what you paid for your NA and what Redlines are going for in your area, you may or may not find it cheaper to sell your car and go get a Redline. Nothing I talk about below will give you any more power than a stock Redline. As a matter of fact, none of it will even get you close.
That said, there are some things you can do to eek out a little more power.
First is a muffler swap. You can go with the Magnaflow muffler designed for the 2002 Camaros and put it in keeping your stock piping. Our stock exhaust piping is actually pretty good. You will need a exhaust shop to weld it up and you can, if you want, add a second outlet to it. For inexpensive though, just keep your single outlet and have the muffler shop weld up that tip to the new muffler. You may get about 5-10 whp out of this.
Cold Air Intakes. You will actually lose a little whp with this one...with the GMPP intake I saw a 1 whp drop for peak wheel horsepower. HOWEVER, you will pick up power in the low to mid ranges and this really is worth the 1whp drop at peak. You'll never miss that but you'll like the pickup you get when you get on the throttle to pass someone or accelerate out of a corner.
High Flow Cat or Catless downpipe. IF you're in CA and don't want check engine lights, ignore this one. You may pick up 5-10 whp here too if you go this route.
Tune. This is going to pick you up the most power but it probably will cost the most as a single item. Figure about 10-15 whp increase with a tune.
Tune and injectors. I list this separately just because you can't do injectors without a tune but you can do a tune with injectors. The stock 2.4 injectors are very small. They are the same sized used on the 2.2 liter. They are 24lb injectors. You can upgrade these injectors to 36lb units and then a tune. RPM (who sells the kit for this) has seen as high as a 26 whp increase doing this, a catless downpipe and intake.
All that combined will cost you at close to half way to a turbo kit yet you're going to only improve 30-35 whp. (Injectors and tune with downpipe and CAI is almost $1,000. Figure another $400 for a muffler installed by a shop) That will take you from about 135 whp stock (the 2.4 is rated at 177 at the crank but dynos about 135 at the wheels) to about 165-170 at the wheels. A stock Redline dynos around 225-235 at the wheels. The RPM K04 turbo kit ($3100) will take you to 225-235. The Stage II kit with the upgraded K04 ($3700) will take you to where tuned Redlines usually sit at (270-280 whp).
So the question is what do you consider cheap and what are your expectations? If you got your 2.4 for under $3-5k, then putting a turbo on it may be cheaper than going out and buying a stock Redline. If you're not wanting Redline power, then a muffler and intake may wake up the mid range enough for you. You may not want the hassle of upgrading things and going with a Redline might suit you more.