Where is the best place to get the tech inspection done? Private garage? Dealer? Others? And what would be a reasonable cost for a 2013 E92 M3?
At my shop we charge an hour to track inspections. Depending on what your dealer charges per hour will decide how much they will charge.
Whenever I need one, I just use the same mechanic that does the work on all of our dump trucks and pick ups at the nursery. If you ask around, you might find you have some sort of "connection" to somebody that would be willing to throw an eyeball on your stuff and sign you off. Don't cut corners though, make sure it's done properly. Sometimes mechanics know to look for things that you can't - and you might learn something in the process!