I think some people are focusing too much on the fact that the IE600 & IE900 use the same driver. Literally hundreds of IEMs use the same Sonion/Knowles drivers with wildly varying prices between them. The fact that Sennheiser developed the driver in-house drives up costs even more, not to mention the extra chamber, tuning & R&D costs, etc. The IE900 was also hit with a price increase due to supply chain issues shortly after launch, prior to the IE600 launch. I'm sure the IE600 was affected as well, but they chose not to increase the price so that created a larger price gap between the two IEMs.