I recently was asked how to do this, with more detail, on Reddit. Here's what I put there (with some additional infos):
Transmitter, dipole antenna and cable $3000 Package here: http://progressive-concepts.com/info/item.php?id=812
Mast assembly $150 (base and poles):
Insulated pivot base assembly
Military antenna mast support poles (4ft each X 12):
Guy-wire and stakes (home depot): $50
Laptop with USB Mixer and 2 mics
Laptop (any will do): $400
Behringer 1204BUSB Mixer $200
2 Shure SM57 Mics with Stands (but any mics will do): $275
This is all you need to get going.
If you want a really small antenna for stealth, go with this 1/4 wave $60:
The brain dead way to tune your 1/4 wave antenna is here:
I have more gear (which is why I said $7800 on the Reddit post) but this is all you need to get on the air.
Remember that it's PIRATE radio.... against FCC regulations. In NJ and Florida it's against the law, but in the rest of the country, it's just breaking an FCC regulations.
The FCC has an enforcement branch, but they don't have alot of manpower. What they usually do is give you a warning and if you turn if off, they go away. If you keep getting busted by them though, you'll eventually get a fine (up to $11,000), but, they have no court to try you in other than the existing court system so they have to get one of their overworked lawyers to convince a court to force you to pay which they hate, so, they try everything possible to get you to just turn it off.
We ran Boulder Free Radio (KBFR) for 5 years before we stopped.
Here's a blog I did on my exploits:
The story of KBFR: