Comcast in Nashville doesn't seem to have a clear policy about CableCARD costs, at least one that all the CSRs understand. I was being charged $15.95 for digital service, which includes the first card, and $6.95 for the second card. I called and pointed out the Comcast FAQ quoted in the previous post, about the second card costing up to $1.91. The CSR graciously reduced the cost of my second card to $1.50. But without mentioning it, he de-authorized my second card from tuning all the digital standard definition channels. Although I could still get the HD channels, apparently the $1.50 charge was for a more limited channel lineup. I called and got the digital SD channels restored. I haven't received my next bill, but I expect that I'll still be charged $6.95 for the second card.
As far as I can tell, there's no comprehensive listing of the channels available for each level of digital service, at least with regard to CableCARD charges for each level.