The main problem is its prevalence, and the social costs that ensue from the use by everyone of something that would be fairly harmless if, say, only the rich were to use it.

主句：the main problem is its prevalence, and the social costs，其中the main problem是主语；is是系动词作谓语；its prevalence, and the social costs是表语。

that ensure from the use by everyone of something that would be fairly harmless if,say,only the rich were to use it.是that引导的修饰social costs的定语从句，而里面又套了一个that引导的修饰something的定语从句：that would be fairly harmless if,say,only the rich were to use it.

译文：
主要的问题是它的普遍性，以及那些或许仅富人使用而无害的东西用于所有的人而导致的社会成本。

根据fool和shuibingkiev的提醒和建议对译文作了修改，谢谢二位。措辞或还需斟酌，但意思应不错了。