Hi ISNEKO
There is no initial payment given to premium authors (although each contract may be different). Rather, Tapas is kind of giving readers guarantee that these "premium" content will meet a certain threshold (in a perfect world, every premium novel/comic will be worth the readers' money but that's not always the case). As far as payment goes--again, this is only from my experience but it's also a good basis--you receive a portion of the revenue generated by readers who spend money on coins to purchase keys to your novel/comic.
So, rather than relying on ad views and tips from generous readers, for every reader who unlocks an episode to your novel/comic, you will gain a portion of the money those readers spent. It's split between the creator, Tapas, and Apple (they get a handsome amount for just allowing Tapas to have an app on their app store)
I'm sure you've noticed by now, but premium content is publicized more heavily, meaning more exposure and potential revenue, so there's also that plus. While it may seem cruel to say, just speaking directly from a business perspective, Tapas has more incentive to make premium novels/comics become more popular because they also gain revenue from those novels/comics as well.
I kind of went into detail, but let me know if I missed something.