Looks like it costs extra per radio you add to your account. So you're paying X now per month for the car, if we bought a at-home receiver, we'd have to pay extra to add it to your account.
http://www.xmradio.com/whatisxm/plans-pricing.xmcThat blows, frankly. They'd probably get a lot more users if you just paid a monthly rate and could then use it with any number of radios you link to your account.