I don't know if you can subscribe only to the Big Ten Network; you'd need to ask your local Comcast customer service people. Here in the Seattle are, BTN is one of the channels you get with the "Sports and Entertainment" package. The Comcast.net channel lineup for your area/zip code should have it listed if it is available at all.
I say "if" becasue it may not be available in your area. We cannot get either the ACC or SEC network here (either or which I'd prefer personally as an uprooted Southerner).
I can't imagine watching sports in non hd this day in age. Non hd channels look really bad on my hd tv. If big ten was available in hd I would sign up in a second. Until then I'll have to pass. Go bearcats! Next week I won't be able to watch u smash ohio state.