They can be built cheaper, but considering the time involved, those are ALL great bargains.. VERY effective!!
I can attest to the use of such 'tater guns'. Mine uses an irrigation valve and a blow gun trigger to 'fire' it (see pic). It'll launch it's PVC projectile 500Ft away, given enough elevation.
I "weighted" my bullets with lead sinkers.... BAD DASS! The Zebco 808 fishin' reel fits on the end of the barrel and is loaded with PowerPro 100lb test.
About 6 years ago, it cost me $66 to build and took about 5 hours. So the prices these guys are getting are fair IMHO.
BTW, note the threaded coupling I used on the breach of the barrel..... Miine is a "takedown" model lol