There have been several fairly recent price drops on early titles in Fiona Buckley's (pen name for Valerie Anand) Ursula Blanchard series, which is one of my really favorite historical series. But one whose titles are mostly very very expensive when not on sale - although with patience you can eventually get them all, in the US at least. (Several of the older ones seem not to be available in e-book in the UK.) All are $1.99 each, except the first, which is $2.99.
To Shield the Queen, #1, apa
The Robsart Mystery
Kindle US:
https://www.amazon.com/dp/B00BOTY2JQ
Kobo US:
https://www.kobo.com/us/en/ebook/to-shield-the-queen
The Doublet Affair, #2
Kindle US:
https://www.amazon.com/dp/B00BOTY2KA
Kobo US:
https://www.kobo.com/us/en/ebook/the-doublet-affair
A Pawn for a Queen, #6
Kindle US:
https://www.amazon.com/dp/B00C8UJIMQ
Kobo US:
https://www.kobo.com/us/en/ebook/a-pawn-for-a-queen
The Fugitive Queen, #7
Kindle US:
https://www.amazon.com/dp/B00BSA2VGM
Kobo US:
https://www.kobo.com/us/en/ebook/the-fugitive-queen
The Siren Queen, #8
Kindle US:
https://www.amazon.com/dp/B00BOTY2IW
Kobo US:
https://www.kobo.com/us/en/ebook/the-siren-queen