As of 1:00 PM on 3/2/18, Prepper Gun Shop had 6 of these triggers for $420 each. To save $29 you might want to check them out first.:

The revolutionary Franklin Armory Binary Firing System converts an AR-15 to fire one round with each function of the trigger, whether a pull or a release. The three-position selector has three modes: safe, traditional semi automatic, and binary fire. To negate the round that would be fired on release, simply hold the trigger back and move the selector back to semi automatic mode, then release the trigger. Installation is easy with no custom fitting required.

Specifications and Features
Franklin Armory 5550
Converts AR-15 to fire one round on each trigger function
Three position selector
Works in all AR applications
Release phase has positive reset
Backup disconnector eliminated
Stronger buffer spring reduces possibility of hammer follow
Easy installation, no custom fitting required

818725010859 – binary trigger system