Whats the charge to keep VPN after 8/31?

You mean the built-in VPN? It will be free.

Need to know what you intend to charge for VPN SERVICES

From June 9th to August 31st users will have unlimited data usage for the built-in VPN in desktop Firefox. From September 1st and on the the data usage will resume again with a 50GB limit for each month like it was before June. https://support.mozilla.org/en-US/kb/built-in-vpn#w_manage-website-settings

If you need to continue having unlimited data usage then Mozilla has their own standalone subscription based Mozilla VPN app https://vpn.mozilla.org

Though you posted this question in Firefox for Android section and the Firefox for Android does now have a built-in VPN in a experimental progressive rollout testing. https://support.mozilla.org/en-US/kb/mobile-android-vpn