Since you're asking opinions...
Take the CAI you have and put it on eBay. Put back the stock setup with a
K&N filter and remove the air silencer. That thing is nothing but bling on an otherwise stock car. That'll add a bit more to your budget.
Gears. You don't have anything about your trans in your profile, but the total cost to install may be a bit more than $200 that somebody alluded to. The SpeedCal alone is going to cost you $100 to your door if you buy new. If you have an auto, you're looking at either a hand held or a chip to correct speedo and shift points. For either, count on $300. Gear prices have recently gone up (at least for FRPP), so count on $200 to your door. Labor rates are going to depend on where you live, and the number of shops that do them. Far and awaythe biggest bang for the buck in a Mustang.
Springs. Do it right the first time. Use a quality progressive rate spring, and install 4 bolt caster/
camber plates at the same time, even if you don't absolutely need them for the springs. Believe me, it'll help down the road. Springs will run about $200 to your door, with another $200 for the plates. Just resist the temptation to go too low if you drive the car a lot. Speed bumps and your exhaust system do not mix well. As with gears, install costs are dependant on your local market.
Subframe connectors. Nope, you won't go one bit faster, but it sure will tighten your car up an amazing amount. If you plan to keep your car any length of time, you're doing yourself a favor to do this first. Unless something flat out comes loose, you won't be plagued with rattles and noises over time from all of the constant chassis flex. I got mine done by a local shop that make them (it's just rectangular steel stock) and installed for $125.
Exhaust. There is no "one size fit's all". It depends on what you want. Little bit louder? Find a pair of mufflers off of a Bullitt. Bit louder yet?
Magnaflow cat back. Obnoxiously loud to neighbors and law enforecement? Off road X and either Magnapacks or SLP. If you live in a state/locality with emissions inspectoin, go with a catted X, H or Prochamber and save the hassles of swapping the thing out every year or 2. Expect nothing more from the exhaust than sound, and you won't be disappointed. If you want a big performance increase, don't bother. Dollar for dollar, not money well spent to go fast on an otherwise stock engine. For those that displute that, please post timeslips (not dyno numbers) from the same day, within 2 hours of one another with nothing but the exhaust changes.
Finally, whatever you do, do things one at a time, and live with them for some weeks or a month. If any problems crop up, you'll be time and $$ ahead on diagnosing where the problem came from. If you dump a bunch of mods on the car all at once, you can drive yourself crazy trying to determine which mod is the problem.
Good luck!
Jerry
don't know where you are getting that info from, but it's wrong. my stock catback showed signs of rust within 1 year. my Magnaflow is going on 2 years with absolutely no rust whatsoever. with the exception of some road grime, it looks just like the day it was installed. why would 
