Either way, it's really not worth the time and effort to set it up right now unless you're a huge site like wikipedia and you don't have any ad revenue to lose.
I do think it will be interesting what this does to donations to wikipedia. It would make sense, I think, that Brave users may donate less per user overall as they feel they are contributing by using Brave.
https://twitter.com/BrendanEich/status/1155334969583030272
Quoting:
"Looks like 26 people donated 50.35BAT in 25 days. comScore has 100 page/day browsing average. Assume 1<=N<100 http://aternos.com pages per day for all 26 ppl & all unblock ads there: at most .65N x PageCPM <> 50.35 BAT or PageCPM <> 77.46BAT / N =~ $20/N. I believe BAT wins."
Again, disclaimers: rough, small-N, lots of other problems. We will do proper case studies as we can. But assuming we would max out at $300 to you is unjustified.
It'd be quite interesting to know what your aggregate CPM is. One way you could probably determine brave users is by contrasting raw user agents against your analytics data. The discrepancy there is going to be some mixture of Brave and those running other things that also block analytics like uBlock. But it should at least you give you a very rough ballpark CPM that's probably going to be much more reliable than e.g. a survey.
One other important figure that Brave could probably roughly answer (but also probably won't) would be what percent of Brave users opt in ads.
Sure it's only a few bucks a month but look at that in terms of your user base. What are you going to assume your marketshare of Brave users is? For context consider the marketshare of Firefox is in the single digits. Brave is probably a fraction of that. If we assume Brave's popularity is 10% of that of Firefox, that'd give you an expected userbase of < 1% Brave users.
That'd be $3/month for, at most, 10k users. If your entire userbase was on brave that'd be $300/month for doing literally nothing. It's really quite a nice model.
Actually this also understates the profit per person for another reason. In Brave you have to opt in to the site rewards system. So you're likely not only seeing a fraction of your userbase paying you but a fraction of your Brave users.