Mine came with one, apparently a guy ordered it on his but didnt like the way it looked when his bike showed up. They swapped it with the stock seat from mine before I bought it, his loss was my gain.
I LOVE IT. Its actually the first BMW seat I have ever sat on that is actually comfortable! I did three 200 mile days in a row to get it back for the 600 mile service ASAP, each day was great and I didnt want to get off. There have been a few longer days since as well. I am shocked quite honestly, I was not aware BMW was capable of making a comfortable seat.
I would check that price though, if ordered separately thats a $450 seat, is that in place of the standard seat? If they are selling them for $150, I want an extra.