Will Plone cost money in the future for either clients or developers?
No. Should the Foundation offer a non-GPL-licensed version of Plone, then commercially licensed versions of Plone would be able to do this, but they would be products separate from the Plone you are currently using. A company may choose to license the Plone source code and sell it as a separate product, but the Plone you are using now will always be under an Open Source license.