The first five books of Jennifer Estep's Elemental Assassin urban fantasy series (published by Pocket Books / Simon and Schuster) have been $2.99 each on Amazon (for US customers) for a couple of weeks now and are still on sale, starting with
Spider's Bite. (The others are Web of Lies, Tangled Threads, Venom and Spider's Revenge.)
Also in urban fantasy, the first three books in the Horngate Witches series by Diana Pharaoh Francis (also published by Pocket Books / Simon and Schuster) are currently $1.99 each (for US customers), starting with
Bitter Night. (The next two books are Crimson Wind and Shadow City.)