Muse is a pre-launch software available at no charge during the beta period. At the end of the beta period (early 2012), Adobe® will offer for-purchase subscriptions which are currently expected to be US $15-$20/month or US $180-$240/year.
If at some time in the future you would like to make changes to your Muse web site, you will need an active subscription to Muse in order to open the design, update it, then re-publish it to your web hosting provider.
I'm not too hopeful for this app. While the entire podcast is nifty, the last few minutes of it address Muse and the problems with Muse in current and future web design.