I really like XM and feel like the $12.95 per month is worth it (especially when compared to the cost of cable tv - and that is essentially what XM radio is - "cable" radio). If you add an additional unit it is $6.99 a month, but you still need to pay the $12.95 a month for the original radio. Or, you can do a prepaid plan for a few months and get a little bit off.
http://www.xmradio.com/shop/subscriptions.xmc
I don't really think Nissan should get a special rate not offered to others. Most car manufacturers now offer either XM or Sirius in their automobiles. You mentioned something about signing up without an "incentive" - I think the incentive is that you receive 3 months of XM free after the purchase of your new car. After that it is up to the driver to decide if they would like to continue their subscription or not. I have been with XM for about five years and feel like the money that I have given to them over the years has been well-spent.