The post links to an inexpensive array at the end. I don't really get why the 16-mic one he used is so expensive, those smd mics can't be more than $1 or so each...
The actual mic capsules are likly far cheaper than $1 a piece (probably closer to $0.10 than $1) but the mics in an array need to be phased-matched. The two approaches to getting phased microphones are 1) building them using precision techniques so they are phased-match from the start (which is expensive and why pro phase-matched mics are around $1,000 each), or 2) get a whole pile of cheap mic, test them one-by-one (or really, pair-by-pair) and select the mics that are best phased-match to use them in the array. The #2 approach is cheaper, but does add cost.
I've never used phased matched mics in my arrays (can't afford it!) and also have never needed to "bin" them. ("pair-by-pair" testing).
I made a 16 mic array out of a bunch of trash-picked and cheap 4ch ADCs.