Check a site like
http://www.everythingg35.com for an example of cost. Bonus is you should be able to order it in the correct factory color. Then the job is just replacing the cracked piece.
This is a hell of a lot easier than installing a spolier from scratch - which is what the other thread is discussing. This also makes this a possible do it yourself job to save money
Check the site i mentioned and also get a quote from the dealer ( separte price for part and labor.
Good Luck