Spoiler : $100-$150
LS Sport Taillight Covers : $150
Lowering Springs : $250, plus install & alignment.
Painted trim : DIY for price of materials, or pay a shop
Intake: DIY for $50 or pay up to $250 for a FIPK kit.
Headers: $600-$900 plus install
Exhaust: $200 (local shop, crush bent, aluminized steel, no tips)-$1000(Corsa, mandrel bend, stainless, tips) DIY or pay install for kits
Hiflow cat: should probably just scrap the headers and buy a downpipe. $200-$250
I would do the Trim, Debading, Taillight Covers, and Spoiler all at once. You'll probably need at least the spoiler or taillight covers painted, and it'll be cheaper that way.
A lot of the midrange kits are popular for exhaust, running about $500. Easy enough to install by yourself, only basic tools are needed. You'll pick up a few horsepower with a mandrel kit, but nothing to write home about. Otherwise I recommend having a local shop do it. The local shop won't likely have mandrel bends, and you'll pay more for stainless throughout.
Headers aren't really that popular, as I don't think they are compatible with the ZZP downpipe, which is a big seller for the W-bodys (Impala, Grand Prix, Monte Carlo). ZZP downpipe will eliminate the restrictive ubend in the exhaust, but you may run into issues with emissions unless you replace the 02 sensor and (maybe) add an 02 sim, for about 40 bucks.
Are you good at working on cars? Turning your own wrenches will save you a lot of money on installs.