Bookbub is like the holy grail of promos. Very hard to get one since they are extremely picky. Pricing for their featured deals depends on genre and your book's price: https://www.bookbub.com/partners/pricing
Oh well, if that's the case, then you can keep BookBub. I will stick with Facebook. In fact, I have now set up my Author Page on FB, so I will just see how it goes. Some authors claim that, with a bit of regular promotion, a FB business page can really take off. Let's just see anyway.