The amount charged in a Placker paid subscription depends on the number of users added and the selected plan.
There are no free plans and access to Placker features is not included in the Trello Gold or Trello Business Class accounts, the power-up is simply an integration between Trello and Placker.
You can use the free trial though to test and make sure that your team will benefit from the Placker solution.
It is not required to add all members in your Trello boards to Placker, but only the ones that are subscribed will have access to Placker view modes, creating mirrors, importing boards, etc.
Specifically on card mirroring, adding more members can make your limit increase, check this article for more information on this,
As prices may vary over time, please check our pricing page for updated values and terms.