top of the line tivo that records 300 hours of hd with four tuners with lifetime sub is 500. In the long run you save money if your renting a dvr in about 3 years you will recoop your money. If you want a lower end one they are available.
There are no known plans for Comcast to start selling Motorola DVR's. The only Comcast supported device that you can purchase is Tivo. A Tivo with a Comcast supplied cablecard can get On Demand in some markets, with more being added all the time.
Let me add: There are cablecard devices that you can connect to your computer.