Amazon (AMS) ads are a great place to start. If you haven't already, check out Brian Meeks book on AMS ads (https://www.amazon.com/dp/B072SNXYMY)
Start with a small daily budget, monitor your ads daily and see how they convert. Ignore the current suggested bids as they can really be out to lunch.
The problem may not be your ads at all, they could be with your blurb, your cover, your price or your look inside.
Use AMS ads to set a baseline, then if they aren't converting as well as you would like (1 in 10) then consider tweaking each of the above, one at a time, leaving enough time between changes to monitor the impacts. That way you can see if the changes are having a positive impact. It takes a while, but it is worth it to have measurable results.
Great, I'm glad it's useful; although the erotica market is different in some ways to other genre fiction markets (discussed here frequently), many core principles are the same. Some responses:
Hi dan
Facebook ads have only kept getting more expensive since I've made that post, and I hardly ever use them anymore.
I get my best results from amazon ads and ad stacking with book promotion websites now.
This book is one of the best resources for amazon ads.
This article is a great resource for book promotions. Also if you sign up to his email list you'll get another file with the top ROI websites.
The basic idea is to stack as many ads on consecutive days as you can during a launch or kindle countdown deal for a ranking spike, then using optimized keywords and product page to continue selling organically.
I also run a facebook group with about 200 horror/supernatural authors trying to help each other improve and market their work. Publishers are always popular, and it's a great place to swap ideas and promotions.
Cheers,