Bookbub does a teaser initial offer. They aren’t free if more promotion is desired to be bought, and that’s how they make money so more are often bought. $100s per promotional thing.
Eventual money is from audio books in other languages, the downside of Select.
I haven't found any teaser pricing and I'm almost an S.M.E. regarding Bookbub. The BB Featured Deals was $613 for broad US and international exposure. Rather worth it, though, since in only 3 hours I've made 315 sales... about 6 times the amount of sales I made in two-and-a-half months.