Question!!!
 

how much would it be to buy a 20B and instill it into and 2004 RX-8 and what would need to be done for this to be possible?

A lot

$12 to 25k cash, depending on how handy you are.

I typed "20B install" in the SEARCH on top right of page and found lots of info....give the Search a try....

lol so its cheaper jsut to by an FD?

i recently found a 20b for about 6.5k. it was "in great condition" with "low miles". If you bought that, i suppose you'd need some other things too. and then it'd have to all be put in. so imagine it would be around 15k. i may be doing this in about 10 years.

Probably be cheaper to flip the car onto it roof, set it on fire, then repair it.

Agree with the others, just buy an FD...even if you have to rebuild/replace the entire 13B and TT's, you'll be looking at a significantly cheaper bill.