data.world is and will remain free for all members through the end of our preview (beta) release. We don't yet know when we will move into the next phase; right now we're placing extreme emphasis on learning from early members and rapidly improving the platform. When we do move into general release, the small subset of our members who require private datasets or an abnormally large amount of storage will be asked to pay an extremely reasonable fee.
TL;DR: You probably will never pay us for anything. A fraction of our members will eventually pay us for private and/or very large data storage.