My thoughts on this are what if I wanted to like visit some friends in Idaho and take my guns, but I didn't want to drive 22 hours. Perhaps flying with a friend on their private plane would be cheaper.
Not unheard of. May not be any cheaper though.
Given today’s experience conditions it’s 1100NM from Austin to Boise. Call it 8+ hrs @ 135 knots and 15 gallons per hour fuel and $5/gal for AVGAS. There will be a fuel/bladder drop for this flight. Probably $600 in gas one way.
The FAA also has something known as common purpose, but so long as your buddy says sure, let’s split the cost, you’re pretty much good to go. But it can quickly get complicated in determining common purpose.