It’ ll cost you double what you paid last time. 它的价钱将是你上次花的钱的两倍。

double the amount/number/size etc He now earns double the amount he used to. 他现在赚的钱是他过去的两倍。

double that (=double that amount) There are only ten children in the class at the moment but it could be double that after Christmas. 那个（数量）的两倍