Yes, I believe eventually relays will charge sats to post and/or read. This allows relay operators to have a continuous revenue stream rather than the present "pay to open an account" model. This will be out of necessity, and allows competition to create the best server. The pay-to-use public relays will incentivize people to 'uncle Jim' their own private relay and will encourage small local communities and better data resiliency and a sense of 'knowing where your data is'.