I believe it's 2400msp, which would be US$30? (Given that 800msp costs $10?)
But yeah I'd say it's actually worth it at that price, it's an exceptional example of how DLC/expansions should be done IMO.
Alternatively you could just shop around for the standalone disc (and install the content, along with KOTN, and sell it off) or potentially pick up the GOTY version of the game if you can find it cheaply
I was wondering about that myself. The 5th anniversary edition just came out for $30.00, it comes with Knights of the nine and the isles. It's cheaper to get that rather than just paying for the MS points.