Bargain @ $0.99-$3.99/£0.99-£2.99 in Canada & the UK from Hodder & Stoughton, who have the legal rights to the late Mary Stewart's (
ISFDB,
Wikipedia) estate (not available in the US, and there has been previous discussion on MR about the apparently pirated omnibus edition called “Legacy”, which keeps popping up in various stores):
The entirety of her classic Merlin quartet, plus the 5th tie-in novel, and some assorted literary novels/romantic suspenses which ISFDB seems to think have sfnal elements (probably gothic). I checked on these a couple of months ago, and they were all at $8.99 CAD, so this is dirt cheap. Linkage goes to Kobo CA, but should be the same price at the other national retailers.
The specific titles in her Merlin sequence are (for linkage for her mysteries, please see the
Mystery megathread where a bunch of other Hodder & Quercus authors are on equal deep-discount sale):
They've also a slew of Tad Williams (
ISFDB,
Wikipedia) fantasy on sale for the same prices (UK only, someone else has his North American rights):
linkage for the lot.