这句话是这样的：the monetary and social costs of allowing the conditions that creat acid rain to continue are potentially very great,AS ARE the costs of available processess to remove the sulfur and nitrogen
网友回答
这句话的两层意思：
1、the monetary and social costs（主语） of allowing the conditions that creat acid rain to continue are potentially very great
2、the costs （主语）of available processess to remove the sulfur and nitrogen oxides at the source,before they enter the air are potentially very great
注意,都有are potentially very great,用as来引导从句,并且谓语are前置倒装,就成了楼主的句子
所以主句主语是costs.从句主语也是costs,当然of后面修饰不一样,是不同的代价
======以下答案可供参考======
供参考答案1:
as引导的是一个非限制性定语从句，在这个从句中，as就是主语。as从含义角度讲，代表的是前面整个一句话所说的意义。
供参考答案2:
Just as the available processes to remove the sulfur and nitrogen oxides at the source before they enter the air are very costly, the monetary and social costs of remaining the condition that acid rain are continually created are potentially very high.
供参考答案3:
其实as此处的意思就相当于so，比方举个例子你一定知道如：I am a student。 So are they.我是一个学生。他们也是。因此此处as的意思是“也是”的意思，表示as后面的情况与其前的情况一样。are是倒装的谓语，其主语是the costs of.....