Smith & Wesson 15-22 $500
Timney Trigger $200
$1150 for a simulated full auto 22 pricey yeah but about 10% of the cost of a registered NFA 22. Plus you can use the stock on other ARs.
I don't have one as I have full autos already, but completely understand why someone would want one. I've shot one on a 9mm as well as 5.56 AR and have to say it was a lot of fun.