The coffee at that shop is stupidly expensive.

stupidly 是形容词 stupid 的副词形式，stupid 本意是"愚蠢的"，在这里用作副词修饰形容词 expensive，表示程度极高，带有夸张和不满的语气。stupidly expensive 可以理解为"贵得离谱"或"贵得荒唐"。