Is the screencast just speech or are there other things on the soundtrack, music, effects, anything like that, particularly over the voices? I’ve used Dragon Naturally Speaking, and the version for the Mac, Dictate, is over $140. Mac OSX has its own built-in voice recognition software but I’ve never use it. But, if you have an iPhone, the people at Nuance, who make Dragon and Dictate, have a free app called Dragon that works pretty well. Voice recognition technology is actually pretty difficult. But the iPhone app works very well, but if the screencast has a lot of background noise it could cause problem but the transcript will require manual tweaking anyway. If you can get major chunks of it that would be something at least.