The Kobo Long Weekend sale in Canada has a quite a number of couponable bargain mystery/thrillers from imprints like Open Road's Mysterious Press, Kensington, ECW Press, etc. at the $1.99-$4.99 mark which also seem to mostly have their prices the same in the US as well (and probably matched in other stores as some of them are publisher multi-store promos, though Kobo does have some sale titles exclusive):
Save up to 70% on bestselling Mysteries & Thrillers! (filtered for couponability; there's also some non-further-discountable $4.99 sale titles from HarperCollins and the like by Peter Robinson, Steig Larsson, Jo Nesbø, Alexander McCall Smith, etc.)
There's some Ed Gorman, Stuart Kaminsky, Joanne Fluke, and Lisa Jackson, and several other well-established popular genre authors in there.
(ETA:
UK store version of the sale doesn't have a dedicated genre page, but there are some couponable mystery/thriller authors at £0.98 including Laura Lippman, Michael Ridpath, Sam Eastland, Nele Neuhaus, and Stephen King)
I personally recommend:
- @ $1.99, Old Bones by Aaron Elkins (SYKM, Wikipedia), the 1988 Edgar Award for Best Mystery-winning 4th in his Gideon Oliver series of forensic anthropology mysteries. This volume, which links a modern murder with some WWII ties, can be read standalone and is really quite good, with a nifty whodunnit. I really enjoyed this when I got it from Fictionwise a couple of years ago.
- @ $4.99, The King of Shanghai (geo-restricted to Canada, they have a different non-couponable publisher in the US), 7th and latest in Ian Hamilton's (SYKM) Arthur Ellis & Barry Award-nominated Ava Lee investigative action mystery series starring a Chinese-Canadian lapsed Catholic lesbian forensic accountant. This is a fairly recent new release which I picked up during the winter holidays, so I'm surprised to see it marked down this far this soon, and was a good continuation of the series with important plot developments. However, you do need to read these in order and unfortunately the previous installments in the series are not being offered on sale this time around.
Not officially part of the Kobo sale, but as a regular rotating markdown by Soho Crime @ $1.99 each (couponable, price-matched in many of the standard Canada & US stores, may or may not also be discounted in other regions, etc.):
- The Steam Pig by the late James McClure (SYKM, Wikipedia), a reprint of the vintage 1971 Gold Dagger Award-winning 1st in his Kramer & Zondi police procedural series starring an Afrikaner detective & a Bantu sergeant in Trekkersburg, South Africa.
- Another Sun by Barry Award-finalist Timothy Williams (SYKM), 1st in his Anne Marie Laveaud series of police procedurals set in 80s/90s formerly-colonial Guadeloupe, starring the eponymous French-Algerian judge.
- Murder in the Marais by Cara Black (SYKM), the Anthony Award-finalist 1st in her Aimee Leduc series starring a Parisian private investigator. This has previously been offered free a very long time ago back in 2009, so it's possible you may already have it, though IIRC the old freebies from Soho back when they were offering them, were in Topaz format at Amazon.
- Random Violence by Jassy Mackenzie (SYKM), 1st in her Jade de Jong series starring a private investigator in Johannesburg, South Africa.
Also bargains from Simon & Schuster UK & Random House's Vintage Canada imprint (neither couponable):
- @ $1.99, City of Fiends by Michael Jecks (SYKM, Wikipedia), 31st in his long-running highly popular Knights Templar series of medieval murder mysteries starring an ex-Crusader returned to England, and his bailiff buddy. You probably don't need to read these in order, as the ones I tried from a previous sale which were mid-series seemed perfectly readable standalone, though YMMV.
- @ $4.99, John Cardinal Mysteries 3-Book Bundle by Giles Blunt (SYKM, Wikipedia), an omnibus of #1-3 in his police procedural thriller series starring the titular Detective set in the fictional town of Algonquin Bay in northern Ontario. The 2nd novel was the 2004 Arthur Ellis Award-winner for Best Mystery, and all the books included have been finalists for some combination of the Ellis, Anthony, Macavity, Hammett, and Silver Dagger Awards, according to SYKM.
ETA: in the UK, on sale for £0.99 from Simon & Schuster UK (not couponable):
Squire Throwleigh's Heir by Michael Jecks, 7th in his Knights Templar series. This has previously been discounted to $1.99 in Canada late last year, so if you travel, you might have picked it up back then. This, incidentally, is the one that I bought and tried, and liked well enough that I'm considering any other reasonably similar Jecks books to be auto-buys at the same $2 mark on sale.