Use the adjective rock-bottom to describe something that can't be any lower, especially a price. If a store is selling TVs at rock-bottom prices, you probably can't find one any cheaper.
You'll almost always find this word describing the low price tag of something that's on sale or that has a very low cost. The rock-bottom prices for books can usually be found online or in a used book store or yard sale, rather than at a new book store. And you'll get rock-bottom prices on groceries if you clip coupons and buy store brands. Rock-bottom was coined sometime in the late nineteenth century.