I got XM for DH for Christmas, but the prices and equipment are very similar to Sirius. The car kit comes with everything you need for listening in the car. The cost depends, but you can find kits as low as $20-30, or up to $200 or so. The monthly subscription is $12.95/month, or you can pay upfront for longer periods of time and save a bit of $$. Installation takes just a few minutes- you can do it yourself.
We also got a home docking kit (comes with a dock for the receiver, antenna, and plug) and it was less than $30. We just move the little receiver unit (about the size of a Blackberry) back and forth from car to house.
One issue we have had- with the receiver, you tune your car radio to a low end station on the dial to transmit the sound out of the speakers. We live in a fairly good size city, and we have a problem with static due to the large number of stations that we receive here. It is really hard to find a frequency that doesn't have a station (or partial station sound) on it. Because of this, we need to purchase a seperate accessory for the car stereo that will cancel out the radio interferance. It needs to plug into the back of the car srereo, and into the satellite radio dock- and that needs to be professionally installed (they need to remove the stereo from the dash to hook everything up). This will add around $70-$80 to the cost- around $30 for the accessory, and $40 for installation. If you live in a more rural area, you might not need this.