twice as much是两倍的意思.
题的意思是如果那售房的女孩坚持以两倍的价钱卖的话,他也会买的,因为他真的需要这套房子.（现在没有用两倍的价钱买）
变成这样：
He would have paid twice as much as the salesgirl wanted for the house if the salesgirl had insisted because he really wanted
题中省略了much 后的as
无twice much as这个情况