If you're thinking about discounts, a BookFunnel promo may be a good avenue. BF readers seem to be more interested in discounted books than full prices ones, but if you're going to discount anyway, you may reach some people through BF that you wouldn't hit otherwise. In any case, conversion rates (from clicks on your book to sales) are higher than AMS ads, for example, and I often move more books in BF than I do with a newsletter promo. For the price, it probably comes closer to a positive ROI than most things I do. And BF is good for other things besides just promos, depending on what your needs are.
If you do go that way, it's important to find a promo with an appropriate theme. That increases the odds that you will get more sales, as BF readers do seem to pay attention to theme as well as genre. Of course, it's also good to have a large number of authors in the promo (more promotional reach). But the more specific the theme, the shorter the list of participants sometimes is, so it may pay to find a good balance between the two.
If you use BF a lot, it's also important to shift among different promo hosts. Sometimes, the same host will run a similar promo each month and attract more or less the same authors. This tends to mean advertising your book(s) to the same audience over and over. The more shifting you can do, the better.
Thanks a lot, Bill. I have the basic BF plan now, and I use it just for getting the ARCs to my ARC team, but I used to have the Mid-list plan, and I did join promos there and (now, this is maybe five years ago) and it seemed that eventually the promos that would be good for my book were pretty scarce, and I got pretty frustrated with it and didn't feel the results justified the time and effort.
But when I think about how frustrating AMS ads are now, my memories of what BF was like seem better. I really think I'm going to give BF another shot.