Let me reply to that question with an image:
Would be nice to make a share of that average monthly revenue.
As you can see, there’s a lot of competition, though, which makes it even more important that you don’t write the wrong things in your books, like using the wrong kind of clothing, hair styles, etc.
To get it right, you need to do research.
Read about the royals, the period, the clothing for the rich and the poor, the politics…
Yes, I admit, that takes time, but it’s necessary to educate yourself before you write. So you either have to do the research…
Or you could simply grab it here http://malka.biz/painless-historical-fiction-regency/