There is no extra charge for HopsFS as part of the Hopsworks platform - so you only pay for what you read/write/store in S3. There is SaaS pricing public on hopsworks.ai.
This was also not clear to me when looking at the pricing page. I assumed there was Free and Enterprise, and that to use this beyond 250 working days, I would have to go to Enterprise.
No, like Dropbox if you do referrals you can get extra credit and stay on the free tier indefinitely. Or you can pay-as-you go on the non-enterprise tier.