Reply to In-ear Monitors: what makes the difference?

I am thoroughly confused in my search for IEMs. I was eyeing the Westone UM1 and UM2 available in the DJTT store, the former of which costs $109. I have been digging around, and have since come across $25 IEMs that have significantly better frequency response than the UM1s do. In fact I realized I already have earbuds that have better frequency response than do "professional" IEMs (granted, the ear-tips are pretty shit and don't isolate as well as they could, but they cost ~$20).
So my question is: what is it that determines the cost of IEMs and what should I be looking for? The critical factor I have personally been looking at in my search for the right IEMs has been frequency response, so that they can be used for both live situations and production. I have been using cans with good and very flat frequency response for the entire time I have been producing, so it would be a shame to lose that. So really, can anyone explain to me why I would choose to spend $109 when I can purchase something with a less prestigious name for $25 and get similar form and potentially better function?