Apple OS updates, whether it be IOS (tablets, phones) or MacOS (desktops, laptops) have always been free.
Your iPad is probably new enough that you’ll be okay for a while. At some point, Apple will stop updating iOS for your particular iPad.
Go to Settings > General > Software Update it should tell you if there is an update you can install.
Please, please, back up your iPad first I always back up my iPhone and my iPad before I install a new version of iOS just to be safe. I’d rather have the backup and not need it over the other way around… wishing I had one