This is a question I hear everyday, sometimes several times a day, over the phone and by e-mail. For us old timers it can get annoying, so bear with us if someone gives you a short answer, and ignor the rude replies. Unfortunately some don't remember they were at the same point, asking the same questions, at one point in time.
To answer such a vague question is very time consuming. As "falcon fanatic" suggested, do some research first then come back with specific questions on things you don't understand. Here's some places to start.
Search the forum. We have hundreds, if not thousands, of topics covering this subject.
Purchase the Falcon Six Performance Handbook.
http://classicinlines.com/proddetail.as ... D200%2DFSH
Read through the various tech articles on my website.
http://classicinlines.com/tech.asp
Read the Q&A section. Its not complete yet, but has a lot of the basic questions.
http://classicinlines.com/Q&A.asp
Look for member websites, as many have good info posted. Here's an example:
http://www.geocities.com/mustang_man_1966/index.html
You don't need a lot of money to do some modifications, but in general it will cost considerably more to build a performance six vrs a V8. Much depends on the modifications, and your experience level (how much you can do yourself).
The basics would include installing headers, an upgraded cam, a 2V conversion, and a better ignition system. Parts will run around $1000-1500 depending on your choice of parts and the options (ie: ceramic coated headers). Labor cost can vary greatly, depending on how much you can do yourself, or how much you spend to have it done by someone else. For example, one of our members built a turbocharged 250ci for $1500 total, while others have spent close to six grand or more for their motors.
Here's a good topic to read, as it will give you a good insight to our insanity.
http://fordsix.com/forum/viewtopic.php?t=49538
That says "Nitrous" its not even spelled the same?? 
