高中英语

as 是对的。
as is often reported,是非限制性定语从句，是修饰The prices of houses have been going up sharply in the past few months.整个句子的，而不是修饰 prices的，意思是“正如（媒体）经常报道的那样。。。”，这个定语从句，还可以前置，变成：As is often reported,the prices of houses have been going up sharply in the past few months.而which 虽然也可引导非限制性定语从句，但它没有“正如”这个意思的，翻译起来比较生硬,它引导的定语从句也不能前置。

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as在这边的用法相当于which指代前面的价格，做句子的成分，同时它还有正如的意思。
比which要形象很多。

句子的意思。房价正如经常报道的，在过去几个月上涨的很厉害

希望能帮到你。

这是习惯用法，虽然都能引导定语从句，但一些习语中多用as
As anybody can see/ as is known to all 众所周知的
As was expected 正如所料
As has been said before 如上所述的
As he pointed out 正如他所指出的

as is reported在这里是做插入语，为一个固定的词组吧，翻译为，据报道。这里不是非限制性定语从句。