The Caveman by Norwegian author Jørn Lier Horst is £1.99 at UK retailers.
This is #9 in the William Wisting series, but the English translations started at #6. I've read the previous three, they are good Nordic police procedurals that seem to be getting better as the series progesses, so I'm looking forward to this one.
I'll give the Apple links here because they have it discounted in other countries as well:
UK £1.99,
IE €2.49,
US $2.99,
AU $3.99,
NZ $4.99. (Not Canada for some reason.)