Where do you get 8 figures from?
I'd have pegged this husband & wife team as solid mid-6.
How do you find out how much authors make?
You don't.
But you can get a feel for it based on how fast they release, and what rank they normally release to in the US.
Anderle has said he's 7 figures a few years ago now, and he releases multiple books a month, with a lot of them making it to top 200 on debut.
Anyone doing 12 releases a year, and getting into the top 200 on the US Paid store with a 4.99 or higher price most of the time, is making mid 6 figures, assuming they're not spending insane amounts on advertising to do it. [Might be less for Trad authors, but the paperbacks will probably make up any difference.]
I base this on what I make, given I'm a top 2000 debut normal. Plus I know what I made in 2018 when I went to 218, 132, and 97.
Anyone who releases enough times a year and consistently debuts at 500 or better, is making 6 figures.
I'm not sure exactly how many Illona Andrews are putting out a year, but they publish multiple very popular series at the same time. So if they're not making mid-6 figures, I'd be asking why not.