Leverage, I was in the same situation...now has changed. Son now pays Dad $100 for very large room and house...and me $50 for healthy dinners and a second place to stay that is very clean (lol) and all washing done.
He is very happy to pay that. Remember that just to share a house costs $130 to $150 then they have to pay all their food, elec. gas etc.
I had the same Q here a while back and everyone was great....$100 when he was here full time was OK but I did not pay his Uni fees so $130 is fantastic value.
Any you can spare, save it for him or just have a great bottle of bubbly.
