In this paper, we prove some of Rubio de Francia's extrapolation results for the class $ B_p$ of weights for which the Hardy operator is bounded on $ L^p(w)$ restricted to decreasing functions. Applications to the boundedness of operators on $ L^p_{{\rm dec}}(w)$ are given. We also present an extension to the $ B_\infty$ case and some connections with classical $ A_p$ theory.