NewbieWar

Why are some more expensive, or why are some illegal and some arent

and what do headers add performance wise

Got-LT1

Some are not street legel because they do not comply with emissions levels in some states. Some cost more thn others for brand name, features, etc. Short tube headers (shortties) will ge you about 15 horsepower, while Long tubes (LTs) will get about 25+. You can also try a search on the subject, and find a lot of info.

EviLBoX

Whether or not they come coated is another big part that determines the price of headers. The better quality headers also tend to have a thicker flange.