Is the charge for eMC Pro one time or annual subscription?
It is a one time charge, but it is also specific to the version. If you purchase a license now, you can use it with any version 7.x. That license will not work on version 8.x when it is released late next year.
However, you can add a lifetime upgrade when you purchase the license or purchase it separately at a later time. That will enable you to use the license with the current and all future versions.