Here: Discourse pricing | Discourse - Civilized Discussion
Are “staff users” the same as “admins & moderators”? If so that’s confusing.
Regardless: I’m good with the Standard plan, but I want 20 moderators, is that $100 / mo (incl. 5 mods) and then 3 x $50 / mo (for an additional 15 mods)? Seems like needing more $$$ to increase the safety of a community isn’t a great incentive model 
Welcome to Meta Dave! 
The short answer is yes they are the same. Staff are either admins or moderators. They are the same according to Understanding user statuses, roles, and permissions.
To save money you can perhaps look at utilising TL4 (Leader) as they do not count towards this staff limit. Furthermore, you can explore with category group moderators
To compare what privileges TL4, group moderation and staff have refer to the topic below
Hope this helps 
Hi Dave. 
@ondrej covered everything well, but I wanted to follow up as well.
Sorry it’s confusing. We try to communicate clearly at all times, so I’ll let our team know your feedback.
I see it says, “Extra staff users” in the add-ons, that must be where you are reading that.
We are about Civilized Discourse, so of course we want your community to remain safe. 
The add-on you are looking at is not designed for your use case. In the link shared (Trust Level Permissions Table (inc Moderator Roles)), you’ll want to check the “Category Moderator” column to see if that permissions group will suffice for your needs.
Let us know if you any questions. 