Actually, I do have a friend with a Ferrari, he picked me up in it, took me to the airfield and we went flying in my plane.
He paid half of the flying, as an exPPl he knows the cost. I didn't pay for the Ferrari trip. Why not? I suppose that cars are more accessible than aircraft, pax expect to pay to fly and not when they are driven. Had we been on a long run in the Ferrari, then I would have offered to pay.
Is it unreasonable to ask for a contribution, I think not. By flying people, we are using our hard earned skills to give passenger an experience that they cannot achieve by themselves. So it is not unreasonable to ask for payment. No one that I have yet flown has begrudged paying me and yes, my passengers have funded my hours-building but I have no ambition to fly commercially.
The problem arises if you start 'touting for business', where you start saying 'Come fly with me - it will cost this much'
I think there is nothing wrong in declaring, 'Yes I have a PPL, I'll take passengers but it costs this much'. That way, if someone wants to fly, they approach you knowing from the beginning that it will cost them.