This is actually free for personal, non-commercial use. It has some limitations on the number of concurrent users, but it’s a great way to host a small home server without spending a dime.
Furthermore, the pricing model for Xlight is typically perpetual, meaning users pay a one-time fee for the software version. However, upgrading to a newer major version (e.g., moving from version 3.x to 4.x) may require an upgrade fee or the purchase of a new license key. Licenses are often sold based on the number of concurrent connections allowed (e.g., 10-user, 50-user, unlimited-user licenses). xlight ftp server license key