Under GDPR, if your data processing is based on user consent, then that consent must be given freely. That means you should provide equivalent service, regardless of consent being given. Providing one service for free and another for a fee is not equivalent, so I'd argue that such consent would be invalid, as it was not freely given.
However, consent is only one of several conditions for processing data. Another would be "legitimate interests" of the business. If you choose to process data based on it, then you do not need consent at all. In such case you can provide separate services: free with data sharing, and paid without data sharing.
The trick here is that you actually need to be able to prove that "legitimate interests" of your business require collection and processing of that data. And additionally such interests are overridden by "fundamental rights and freedoms" of users. Which is probably why most businesses went the consent route, as it appears to be more clear-cut.
Thanks a lot for the insight, wasn't familiar with the GDPR logic. I think it'd be very hard to make a business case for processing non-paying users only so it's pretty easy to understand the consent option now.
