Yes, but there is now an option for CPC. It's new. My effective CPC rate under the CPM system was about a dollar per click (obscene) and it is about 30 cents per click under the CPC option. Still ridiculous but slightly more palatable.
Where is it? I just went into my Bookbub account and I still only have the bid on impressions option.
As for the OP, I had an international-only in a small category in January. I think I paid $50 and had 250 sales at 99 cents but I was also running FB, AMS, and Bookbub click ads. I do recall thinking that the ad had paid for itself within 36 hours since the click ads were focused on the U.S. market. (It took until the day after the promo for my Australian sales to register on the non-Amazon sites.)
I have another international-only in about a week but in a larger category. That will cost $174 and is expected to generate 750 or so sales according to their estimates.
From what I've seen people say, probably 95% of the Bookbub newsletter promos are profitable. The challenge is getting one. Being wide instead of in KU and pricing higher are two ways to boost your chances. Also being willing to accept an international-only deal helps, too.
In terms of the click ads, I've been pleased with the ones I've run but I rarely have my books at free or 99 cents so I don't run them often. I think I did try them on a full-priced book and had no real results. (But it was a strategy to get awareness of the book while it was at full-price and before a price promo.)