Several sets of older stuff have recently been put up on Kindle for 99 cents. An example:
https://www.amazon.com/THRILLERS-absolutely-gripping-adventures-Historical-ebook/dp/B0CVNJRDZL/ref=zg_bs_g_7588740011_d_sccl_7/144-3661450-1762052?psc=1
If you scroll down or do a bit of searching, you'll find several multibook sets in different genres for the same price...
