On December 4th I delivered a Webinar on Extranets in SharePoint 2013, and also spoke to the licensing changes that will really change the game for both Extranets and public web sites. For on-premise installations, Microsoft is eliminating the For Internet Sites license requirement, bringing the licensing costs down significantly. Likewise in the cloud, Microsoft now provides up to 10,000 external user accounts for Extranets in Office 365, with no additional subscription fees.
More details about the webinar as well as links to the presentation and video can be found below
Presentation - Extranets in SharePoint 2013
Video - Extranets in SharePoint 2013