theyrnt expensive!!! go to the istallation guide an check the link. its like 28-30 for the complete LED set!!
Depends on what colors you use. Blue led's are over $1 a piece, while Red ones run you around 5-10 cents per, I can't remember exactly.
I only used blue LED's for the LCD display (gas, temperature, mileage) and used red for the rest.
As for the install, there really isn't a way that you could mess it up, unless you really abused the board.