Being the nerd I am, I broke down the cost of downloading every episode of every show I watch for a whole year. This only includes episodes of shows I have season passes for on my TiVo. It does not include any sports or shows that I happen to watch if they're on (*ahem* Project Runway). Also worth noting is that I estimated the number of episodes based on previous seasons.
As of right now, my yearly cable bill comes to $648 ($54 a month). If I were to download my shows online, it would cost $1,140, which is a difference of $492, or $41 a month. The difference isn't ridiculous, but if you tack on the cost of watching live television (sports, awards shows, etc.) it will add up quickly.
I'm not sure how the pricing would work there, but I currently pay about $200 a year for NFL Sunday Ticket, which is every NFL game (that amount was not included in the $648).
If the cost initially seems negligible, consider the fact that I live with three other people, who also have shows recording on TiVo. If I had included their shows in the total, the total would have been a few hundred dollars higher, easily.
I won't get into the possibility of sharing downloads as that's a whole post unto itself.
-"The cost of a la carte television" Capndesign.com, Jan. 30