From https://developer.apple.com/support/transition/
The development resources available in the Safari Developer Program are now part of the new single Apple Developer Program. Your existing Safari Developer Program membership will remain active until July 8, 2015 and your Safari extensions will continue to work for existing users. Your extensions will also remain available for download on the existing Safari Extensions Gallery until the fall.
In early fall, the new Safari Extensions Gallery for OS X El Capitan will go live. This gallery will be the safest and most reliable place for users to download Safari Extensions, as all extensions will be signed and hosted by Apple to ensure that they are safe to install. All updates to your existing extensions, must be submitted to the new gallery.
Why does the Safari page still say that it's free? Probably an oversight.
Because it used to be a separate, free program, but on June 8th all of the Developer programs got restructured.