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。