Diaphragm control (by the amplifier, yes, but also by the 'motor' of the transducers themselves) would also affect the ability to convey multiple aspects of sound simultaneously (e.g. harmonics, timbre, etc.). On the costings side of that, in addition to proper engineering design, the strength of the magnets can be important, and some scenarios require more expensive rare-earth (neodymium, etc.) types of magnet.
In other words, if you have a cheap motor design for the transducers (e.g. BA or DD), then it may not exert enough control over the diaphragm to convey nuances at the same time as more prominent sounds in the music. In the case of DDs, the material chosen for the diaphragm itself (and for the diaphragm-surround) can have a massive influence on how well damped the diaphragm is, because if it is poorly designed, then all manner of unwanted resonances can potentially over-excite the diaphragm, and can thereby smear important nuances in the recording.
If a designer chooses to use exotic diaphragm materials (beryllium, titanium, etc.), then, sometimes it can increase costs quite dramatically, not necessarily for the material itself, but for the manufacturing process(es) required in order to successfully manipulate the chosen material (titanium is notorious for this, in the bike industry, for example).
Then there's how well designed the IEM enclosure/housing is designed, in relation to the drivers it contains. Loudspeaker designers take great care to choose the volume and shape of their enclosures, as these parameters can have numerous important effects upon the behaviour of the transducers/drive-units, including how well-damped they are. I can relate, from personal experience, that the FutureSonics MG6Pro is guilty of overlooking the importance of this, and suffers badly because of it (well, my pair certainly do).
Basically, there's a whole heap of reasons why cheaper IEMs may not convey timbre as well as more expensive ones, even though it is not always about cost - the skill of the designer in choosing materials and implementing them is every bit as important - actually, more so.
On the fullsize 2ch side of the market, although I feel their press publicity is over-hyped, I nonetheless acknowledge that Elac are achieving some very nice sound, at competitive pricepoints, with their mid-fi loudspeaker designs. That is substantially due to skilled design, for their drive units and for their overall implementations. Many of their competitors are just buying-in generic drive units, which isn't always the best way to compete.
Whilst I'm on that topic, it's nice to see that EE are getting involved with specifying bespoke drivers (both BAs and DDs) for their IEMs (even if they're not necessarily manufacturing them all)