Excuse the super long post but here we go!
First- UD pullies- They are not a must but they do help. SN95 3.8Ls do not have a harmonic balancer and UDPs run $70-85. New Edges have a harmonic balancer and UDPs run $150-175. For the $85($175) you will spend on them new they are worth a couple of horse power. They are not a big gain item. Look for used gears, used just means cheaper.
Your cost is at $85 (for non-harmonically balanced)
Exhuast- Not a real big gainer either unless you have bigger mods to support it. You don’t have to spend an arm and a leg on exhaust. You can go to a muffler shop and have them build you a dual set up cheaper than what any cat-back or after market company can offer after shipping. I paid $250 for my duals with flowmasters. I wouldn’t pay much over that for an exhaust system.
Cost is at $335
T-loc- If you are pushing any power you want to be able to get traction. I’m using a used T-loc that has been rebuilt. I’ve had it in my car now for 5 years under some pretty good abuse and never had a problem. I paid $100 for mine, look for a deal on one I wouldn’t pay any more than $250 for one on a budget.
Gears- Make a choice 3.73’s or 4.10’s. See which ones will be right for you. Gears are something that personally I would not get used. I bought Richmonds new and never had a problem with them for $165. I would not suggest installing your own gears and T-loc unless you really know what you doing so I factored in an average $300 installation fee.
Cost is at $1050 ($165 for gears), ($250 for T-loc), ($300 for installation)
K&N filter- I personally think that a K&N filter in the stock air intake tube is just as good as a CAI. That is just my opinion so I would spend the $25 for the filter and leave it at that.
Cost is at $1075
Cam- You will really notice a big difference in the car with a cam. I am using a 207 cam from RPM and really like it. All in all they are a little tricky to install but nothing that anyone who was taking their time and had a good over view of the procedure with a manual couldn’t do. I paid around $180 for my cam.
Heads- These will make the biggest difference in performance. They are pricy though too. Super Six sells a pretty good set for around $1000. I got mine from Gaston who P&P’s them (from V6power.com) for $500. I really like them and he did a fantastic job. While the heads are off this is the best time to do the cam, no worries about holding the valves up and just makes things simpler to do. I also think putting valve spring on heads is a lot easier when they are not on the car.
Valve springs- You will need to get a set of valve springs to go along with the cam and perhaps some push rods. I suggest the adjustable roller rockers and then you can keep you stock push rods. Expect to pay around $100 for a set of springs.
Gaskets- they are more expensive than you think. For upper and lower intake, head, water pump and front seal along with head bolts it cost me around $250. I got all Fel-pro stuff.
Cost is at $2355 ($750 for heads), ($180 for cam), ($100 for valve springs), ($250 for gaskets)
Roller rockers- They make a big difference over stock rockers and you can tell why when you look at them side by side. I suggest the adjustable rockers.They are aroung $255 for non adjustable and $360 for adjustable.
Cost is at $2715
is the biggest bang for the buck mod there is. With no other mod can you spend $500 and get 100 hp out of it. I like the Zex kits but NX is also a very good brand. You can save a lot of money getting a used kit. Ebay is always full of them and they come up for sale on the boards from time to time. I paid right around $500 for mine and around another $150 for accessories.
Cost is at $3365
Tuner or chip- I like the SCT X-cal. For $420 you can’t get a better deal. Chips can tune as well but you are stuck with what is on the chips and if you change your tune needs they have to be sent off and reprogrammed. The chips run around $350.
Cost is at $3785 ($420 for tuner)
Other misc stuff your going to need along the way like a Maf can be found for cheap as well used. I found my maf for free from a friend. Keep your eyes open for any thing you think you might need. If you can get a part for free and you might not use it you can always sell or trade it for something that you do need.
Auto's may need additional help as well. The addition of a higher stall torque converter would help tons with the aod
04 GT’s went for around $23800
V6 mustangs sold for around $18900
Difference of $4900
The build I described and priced out will have you putting car lengths on GT with ease for around a $1000 less than they paid for their car as is.