Do you think your diminishing returns for that series are because you've exhausted the readers they have for your specific historical fiction genre? I wanted to ask that anyway, and it seemed like a good time.
I didn't say that I have diminishing returns for the series. (Maybe I was talking about another series) Or if I inadvertently indicated that it was a misstatement. As a matter of fact, I don't have diminishing sales for the series; they're pretty amazingly steady. It still sells better than more recent series because of the characters and setting. You can always sell books about King Robert the Bruce, or about his amazing follower the Black Douglas.

What I said was that I am not sure I'd have a good enough RoI to justify having an Int'l only promotion since by far my largest market is the US. I have what I consider decent sales for it in the UK, particularly in Scotland, but I am not sure that would translate into a good enough RoI. It would also mean no combined US & int'l promotion for a while.
If you look at the total sales for the major figure in my genre, Bernard Cornwell, who as of a couple of years ago had sold some 30+ million novels, I suspect I still have room for sales. Now a different question might be whether I have exhausted the readers for the genre who are BB members. I have considered that, but considering the size of their historical fiction list which was about 3 million a year or so ago, I don't think so. Historical fiction is their third-largest list and also I have three HF series that I alternate. Of course, when something like Netflix's
Outlaw King comes out that always helps. The sales pattern for historical fiction is totally different from romance for example and novels can continue to have steady sales for many years.
ETA: And I am working on the first novel in a new series right now.

I think that I will accept it basically as an experiment so see how it does. The cost isn't that high. Unfortunately, I'll be hard-pressed to support it with secondary ads another disadvantage.