What can I add, expect to pay(including install), and hp increase.

I was thinking cobra intake, gt-40 heads, TB. What else could I add to get a decent power increase?

 

Cam. Might as well while you're there.

 

If you're really on a budget, find a wrecked 5.0 Explorer in the junkyard and get the intake and the GT-40P heads off of it. Find one that has the external EGR passage to fit your car. You'll have to buy a set of GT-40P specific headers but, with the money saved on the heads that shouldn't be a problem.
Here's a good article answering most of your questions.

http://www.mustang50magazine.com/tec...ads/index.html

 

I'd probably get a better head set. Sky's the limit man. I'd be yankin that 5.0 and poke and stroke it to 331ci, toss on a set of AFR 185's and a nice lumpy but streetable cam and whatever intake gets you hot. With that combo I'd want something on the lines of a trick flow street heat.

For your stock motor, heads/cam/intake/exhaust. Increased ratio rockers could help but those are often fraught with PTV problems.

Nitrous is your single best friend.