I too wondered how many copies they expect to sell.
Scotland doesn't have a huge population, only a percentage of them will want the book. I cant imagine many people not in Scotland will want a copy.
So my best guess is 9000 copies sold.
And as you say, if more comes out, if she ends up in prison, if her husband ends up in prison, if lavender times do indeed happen. I cant see more people wanting copies.
Soooo will we ever find out how much the advance is?