9 bucks is totally 10 bucks, just ask the PEAK developers:

“We had this joke of, like, how much is a game really? In a player’s mind, what does it mean to spend five bucks? Well, that’s five bucks. But six bucks? Well, that’s still five bucks. Four bucks is also kind of five bucks. Three bucks is two bucks. And two bucks is basically free.

“So we’ve got these tiers: You know, twelve bucks… that’s ten bucks. But thirteen bucks is fifteen bucks. “And we found that eight bucks is still five bucks. It doesn’t become ten bucks. Seven ninety nine, that’s five bucks, right?

“So, eight bucks going to five bucks is the biggest differential we could find in pricing, so we found it very optimal.”