Bang for the buck for me starts at around $150 and is the $150 to $300 range. You can get a lot of really good stuff in this price range.

However, there are better products at a higher price point. The downside is the price isn't really tied to the cost, so you're buying a lot into brand, marketing, and market expectation. However, this does give manufacturers some luxury to do extra, like better materials and construction, better drivers, better accessories, more accessories, better case, etc. It can also buy some engineering and development time. You may put more effort into a flagship product rather than a midrange or low end product with heavier costing constraints.

My top three IEMs I own are $1000 range IEMs. Are they worlds better than my stuff around the $300 range? Not a ton, no. We're talking 3x the price for 10%-20%, and even that percentage is subjective. However, the stuff I have in the $150-$300 range blows the cheaper junk out of the water. Low end stuff is significantly worse. There are a few decent cheap things, but the level of sound quality and performance is always a quite reasonably jump stepping into the real mid tier stuff. I've been in the IEM hobby for 20 years. The cheapest one I own is $100, an old Vsonic GR7. Virtually everything I own or want to own is in the $200 to $1000 range. Again, it's not really a cost thing. It's just how the market's spread out these days. You're just stuck buying a $15 cost to manufacture thing for $200 because...reasons.