Yes. Fluxer is free and open source software under the AGPLv3, and you can run your own instance with no license key, paid tier or reduced edition.
You don't have to pay Fluxer anything to self-host. Your costs are the usual ones for running a website, such as the machine, the domain name and an email service if you want email sign-in, and you pay those costs to the providers you choose.
Is self-hosting Fluxer free?
Yes. The license lets you run, change and share Fluxer's open source software. You don't have to buy anything from Fluxer to install an instance, update it or let people sign up.
Do I need the Operator Pass to self-host?
No. Everything in the Fluxer software works without the Operator Pass. The Operator Pass is an optional one-time purchase that comes with a numbered Operator certificate, and section 3.4 of its terms lists what the pass doesn't include.
Yes, by default. Plutonium is the subscription on fluxer.com, and on a new self-hosted instance every account has premium, with the features and higher limits that Plutonium gives on fluxer.com. As the admin, you can change those limits.
You can also switch the instance's premium mode to mirror. When you do, an account on your instance only has premium if it has a subscription or has redeemed a gift code there, and you can sell those subscriptions with your own payment account.