Three ideas:
- Make them all access free for the first week they're up, then charge for access to the back catalog. See CreativeLive[1] for a good example. This way everyone can measure the quality of video, audio, transcripts, etc so they know what they're getting, and the people who need to learn a specific topic will have a strong incentive to pay. This will encourage you to cover evergreen topics to make your back catalog more valuable
- Flip it and make the new videos available to subscribers for the 1st N months, then free afterwards. This will bias you towards covering the newest, churniest stuff that is causing people pain right now
- Do like RailsCasts and alternate between free and paid videos. Paid subscribers get twice as many videos, everybody still gets something free for marketing etc
Personally I think the first one is your best bet.
It might actually be worthwhile comparing those sites to subscription based techcasts since both are in the business of selling expert knowledge.
One major difference I see is that the selling point of poker sites is that the players are successful so there might be value in a screencast site that focuses more on "rockstar developers" etc.
There's no reason why somebody with a back catalog of 200 episodes who doesn't have desire/bandwidth to record more can't sell access to the back catalog and fairly easily get to $X00,000 per year.
So, encourage someone to download each new video and distribute it via torrent once the paywall goes up. Cool.