I actually think it makes good business sense for Brave to share as little information as possible. Yes, users would like more detail but so would the competition and there are probably fraud related ramifications if they freely provide their algorithms. I consider this information proprietary information and do not expect Brave to provide that type information freely to the public (and competition).
Brave does provide some macro information that can give you a broad sense of how BAT is calculated and distributed on a per user basis. I posted a few links below to some various information sources. There are tons of other information sources out there if you are interested in digging it out including snips from podcasts, community posts here and on reddit, Discord, etc.
I think, for users, the main metric to work from would be the “A full 70% of the revenue Brave earns through these unobtrusive, privacy-preserving ads is shared directly back with users as Brave Rewards” from the FAQs in the Brave Rewards article.
Since Brave is a private corporation, you aren’t really going to know the exact sales dollars. However, there are analyst reports out there and Brave does provide macro information relating to ad metrics, number of users, number of ad campaigns, etc. that you could dig into. You could collate the relevant information from each source and crunch some numbers if you’re really motivated. 
I think the decrease in BAT awards has more to do with the number of users opting into Rewards than anything. The more users the less BAT if sales $$s remain constant and even an increase in sales may not offset the number of new users opting in to Rewards.
Then you have to think about the fraudster accounts… I am sure that is impacting BAT distribution amounts too. Fraudsters are not just stealing from Brave and advertisers, they are stealing from the honest users too!
Again, all this information could be provided by Brave but isn’t and I don’t blame them! If it were my business I wouldn’t share any of this information either. lol I don’t have any facts to support my comments either. Just my opinion. I am definitely not motivated to dig into the information out there and find out if I can verify my opinion! 
Information Sources:
Brave Media Kit
Regional Catalog Viewer
TRANSPARENCY DATA FEED
Mostly for reference to creator fees if you want to dig into creator vs user payouts.