I have Comcast in Chicago, and the CableCARD is free. In fact, I actually get a credit from them for using my own equipment instead of their cable box (I think there might even be a law about this).
What they will charge you for is an "additional outlet fee" if you have more than one tuning device (cable box, CableCARD, etc.) If you can pull it off, the best deal (through Comcast) is to get a DVR that you can also watch live TV through instead of using the cable box...then return the cable box to the provider.
AlBQuirky - I think the biggest problem you're going to have using a 3rd-party DVR is getting the guide data. DVRs need to be able to download the guide data (TV listings) for your provider so it knows what's going to be on and when. The cable provider's DVRs have this baked in...they can download the listings directly from the provider over your cable line. That's one of the reasons TiVos need an internet connection. If you're going to buy 3rd-party, you'll have to find a box that's compatible with your provider and make sure they'll let you use it (Comcast in Chicago will not allow this) because it will have to be registered with the provider like a cable box. The alternative is to find one (like a TiVo) that can use a CableCARD and also connect to your internet connection to download guide data.
I use a network-based capture device (SiliconDust HDHomerun Prime) that takes a CableCARD and then provides the cable channels to any device on my network. I have a computer that's always on that acts as the DVR, and it downloads the guide data from a server over the internet (currently just using Windows Media Center w/ Windows 7). This works out really well for me because I use PCs connected to my TVs for all of my media viewing, but it's not the simplest solution for people that just want a set-top DVR. There are also some good alternatives to Windows Media Center if you don't like it (although after trying them all Windows Media Center works the best since my beloved BeyondTV is no more ). The aforementioned NPVR is a good product, and you can also use it as a recording back-end for XBMC, which has a much nicer user interface.
I used to use a capture device that didn't need a CableCARD, but Comcast in Chicago started encrypting ALL of their channels, so I literally can't get any channels anymore without a cable box or CableCARD. That's why I switched to the HDHomerun Prime. By the way, if you have a home network I highly favor the HDHomerun devices over capture cards...they don't rely on hardware drivers to work because they essentially act as network TV servers. Over the years I've used just about every TV capture device under the sun, and the HDHomerun devices have worked the best and been the most reliable by a pretty large margin.