From what I understand the GDPR says you have to give users a real choice about the usage of their data, without any unreasonable negative repercussions. Having to pay money (at least as much as they are asking for) is such an unacceptable repercussion, no matter how FB might phrase it.
They are allowed to take money or show ads for access, but they can’t couple that decision with the one about the user’s data usage.