I've recently had two done. The first one was on my 150 and cost $800. They found a leak and the transponder needed to be recalibrated. All that took about a week to accomplish but I'm happy with the result.
The second one was on my Mooney. It only took about two hours and the inspector certified the GPS on site. No errors or abnormalities were found so it only cost me $200 even.
Hope that helps.
