GAT Trigger $25 shipped (usually sells for $39).
Here's a bunch of descriptive info I found on the web about the GAT Trigger:
The GAT trigger provides rapid fire by turning a crank which pulls the trigger 4 times for every rotation. The GAT Trigger will fit all guns and is mounted to the trigger guard
A semiautomatic fires one shot with each single pull of the trigger. No matter how fast you pull the trigger, it remains legal as long as the trigger functions singly with each shot. The GAT™ does not alter any of the functions of your gun, it simply fires the gun at a faster speed than you can with your fingers.
The GAT™ Trigger Enhancement System can be used on virtually all semiautomatics including SKS, AK47 variants, Ruger 10/22, AR-15 and many others. It installs over the trigger guard with 4 mounting screws that clamp it in place. Two additional mounting screws are used to lock the GAT to the bottom of the trigger guard.
No modifications are required and the unit can be easily installed and quickly removed. To operate, simply insert your finger into the thimble of the lever and rotate it clockwise. Turn lever smoothly and slowly. When a higher rate of fire is desired, rotate the lever faster. Very little pressure is required to operate the device when it is installed properly.
For single shots, simply insert your finger onto the trigger and fire as normal.
THIS TRIGGER ENHANCEMENT SYSTEM IS A VERY VERSATILE RAPID FIRE DEVICE SIMILIAR TO THE TAC SERIES TRIGGERS.
EASIEST TO INSTALL
MOST DURABLE EVER
DOES NOT OBSTRUCT GUN'S TRIGGER
SHOOTS 600 - 1200 ROUNDS PER MINUTE
SHOOTS SINGLE OR MULTI-BURSTS
GOES FROM ONE GUN TO ANOTHER
BATF LEGAL (COMES WITH CERTIFICATE)
RIGHT OR LEFT HANDED