12K is crap. You only need to replace the bellhousing. As domino_z has pointed out it is $800US. Around $1000shipped. Nissan Aus will sell you one for $1400 if you don't want to be a cheap arse like me. In the US, Nissan used to pay the mechanics for 8hrs labour under warranty to replace it, they have now dropped that down to 4hrs. It can definately be replaced without the motor coming out. You would be looking at $3000 tops to have this job done.
The 2011 and 2012 bellhousings are the same part number and as has been told to me these are the ones that have been re-designed to stop the issue from occuring. Willall do make a fix with shims for around $200 to take up the slack but for me I prefered just to buy a new part than I knew was correct from the beginning. I hope this helps somewhat.