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?

Why did you choose to leave out the make/model of your magical $25 IEM's? And how are you measuring the frequency response?

MEElectronics M6 was one model I found that seems attractive. I looked into many reviews and the reports are overwhelmingly positive in regards to sound quality and even soundstage, more frequently negative when it comes to comfort, but that seems to be an issue for absolutely any kind of IEM. I am only comparing the given frequency responses that are listed in the official manufacturer websites.
http://www.meelec.com/MEElectronics_...e-m6cl-mee.htm (it's under the specifications tab. the website design is pretty shitty. but the freq. resp. is 20Hz-20kHz)
Westone doesn't even have that info on their website because they are sold out, but the frequency response on the UM1 is 40 Hz -16 kHz according to the DJTT store, which I highly trust.

also I left out the mark in the OP because I posed a pretty specific question, not sure if had read it or not:
So my question is: what is it that determines the cost of IEMs and what should I be looking for?
really I was asking what is is that matters in determining what is a good and what is a bad IEM. I put a lot of emphasis on frequency response, but perhaps I am looking at the wrong thing.

Unless there are measurements provided, you can't trust frequency response specs. Sure, they may say it's 20Hz-20kHz, but with how much tolerance? Do they state the tolerance only for the roll-off frequencies, or for the full band? It's better to just trust your ears and try to listen to as many IEMs as possible.

Basically it comes down to better quality components and manufacturing processes.

I guess the difference I found after digging more is that the UM1s, and generally the more expensive DJ monitors have higher sensitivity and higher impedance. Correct me if I am wrong, this means that they can be played at much louder volume without destroying the headphone or distorting the sound. So cheaper headphones like the one I found could not withstand particularly high volume, which could potentially be problematic in the club... i guess? I don't really buy it though - if they isolate well then there is no need to crank up the volume really high, and if you do crank it real high, you aren't protecting your ears any more anyway, so what's the point?

Plus when you get up to the better models you're getting multiple drivers which is really the difference in quality IEM's.