HAve a look at OP, I think he already have springs.If you can find cheaper options for the parts listed above I would also look into suspension parts like a rear swaybar and lowering springs. For example I picked up my catback exhaust for $280 shipped to my door. Also depending on what tune you opt for you can easilly knock $100 of that price. I believe sway bar is ~$175 and springs are in the $200 range.
For bang for the bust the FSWerks Stealth is a good exhaust for $500 and if you are just doing a drop-in and not full intake you might as well go with an AEM Dry Flow for $39 shipped.... K&N owns AEM now so it's basically like a washable, dry K&N.I have kw coilovers right now so I won't need the lowering springs and I can get a tune for 400. I'm wanting an exhaust system just don't which ones are best for the price and same with the best drop in filter?