Good question. I'd also like to know what's the genuine difference between a (decent) budget capsule and a premium one?
Back in the 1950s and 60s I imagine the backplates were made by very skilled precision machining technicians. This would have been very skilled, labor-intensive work. Whereas now, I imagine a cnc machine bangs them out hundreds of times faster and probably to tighter tollerances. So the real difference would be in the time, care and quality control of the skinning.
Some people suggest that many of the Chinese K67 capsules come from the same production line and are graded depending on their measured response. You simply pay more for the 'good' ones. But what's the real difference between those and the $500 ones?
To answer OP, I think the microphone-parts.com ones are fairly priced for what you get, and you can't fault Matt's service. I'd like to hear what others think.