“语法探讨”老师的回答很好,但你不一定能看得懂。你的困惑可能是为什么会有这样的结构,这其实是比较结构的一种省略,在一定语境内,听说双方共知比较对象时,常常后面的比较对象省略掉。
还原后,很可能是
He charged us twice as much as we (had) paid last night.
He charged us twice as much as he charged the locals.
He charged us twice as much as it should have been.