All maintenance upgrades are free. If you purchase a license, you are allowed free updates for the corresponding edition for the same minor version. For example, if you own version 10.0.0, and version 10.0.1 is released, you will receive the update free.
You pay for the software only the first time and get all new versions for free for the product lifetime.
Perpetual licenses don't expire (no subscription fees). VAT/Sales Tax might be added.