The add on unit is under two hundred dollars at Best Buys. I have it in the Excursion, but the unit is really ugly and kind of hard to see. The nice thing is you can take it from car to boat to house, etc with just one subscription.
In the F-350 I added a Sony head unit thatwas XM ready and also would control a CD changer. It came with two remotes and works VERY well for about $350.00.
Go to a good audio shop and you will save money, because if your dealer is like mine, he will take it to the same shop anyway and charge you extra.