will it be free?
I almost never get microsoft points so it might be!

It'll most likely be either 800 msp or 1200 msp (if it's close to the size of the Shivering Isles, expect it to be closer to the 1200 range as Shivering Isles cost 1200 msp when it dropped).

Pricing has not been announced yet. Please use this thread for the DLC discussion:

http://www.gamesas.com/topic/1373537-dawnguard-and-future-dlc-discussion-thread-28/