Good point. I think Garmin would have to do the work for the STC. Seems like cost could be estimated based on existing installations.
I'm sure someone who knows more can weigh in on the cost question.
The least expensive Garmin certified model https://www.garmin.com/en-US/p/604257#overview GFC 500 $8,650 plus the required G5 $2,725 = $11,375 PLUS installation. That would require some serious interest!
Since getting my PA-18, my mission has changed over time. I did not think I would be doing so much long cross-country. But here I am. Tucson to Seattle in a Supercub is fun (22 hours of fun last time...) but fatigue would be a lot less with an autopilot.