ESPN2 is significantly cheaper because Disney wants to make it easy to get ESPN2 in as many homes and subscribers as possible. First there is the additional ad revenue. Then, after some time, subscribers become accustomed to viewing ESPN2, along with it back-up programming use, and now ESPN2 may have some value that subscribers feel they appreciate, and that is the point at which Disney will ask significantly more for ESPN2 when the contracts near an end.
All the media companies do this: get secondary channel in at a low ball rate, then increase the value or it and ask a lot more money or a whole lot of your subscribers won't have our channel and you'll be sorry.
This is where the game of chicken comes in if an MVPD fells there isn't value in that channel, they will drop it, or pay if the MVPD fells it has become a channel it subscribers value.