Hello! I have built a zapper from the zapperplans.com schematic, and it's just as simple (only a little more battery efficiant) as the Hulda Clark schematic. The reason they, and probably other zapper companies can get parts cheap is (most likely) because they buy in bulk. Radio shack sells to the people like you and me who aren't buying a ton of parts and don't want more than a few at a time. Most zapper companies probably get their parts from a supplier (like, maybe the place radio shack gets their parts) and they buy lots of parts at a time. As far as I can tell, it should cost no more than a dollar or two for these companies to make their products from start to finish. As for the zapperplan.com zapper working well, It is the only zapper I reccommend!
Hope this helps! God bless!