I made on back in the early 80's and still have it. I bet with a smart phone, an app with calls, amplifier and speaker, a fella could still make one for under $100. Of course they aren't as compact as the one piece dealys y'all have now.
Well before Radio Shack went bankrupt you could buy a battery powered amplifier, and speaker for about $40, hook it up to an MP3 player or your smart phone, there are quite a few calls as MP3 files on the web, and use that.
http://www.instructables.com/id/Electronic-Predator-Call/