That's because rent of $1.5k is for the whole of a house, whereas the mortgage is only covering whichever part of the house you don't own. My rent would be $3

That's because rent of $1.5k is for the whole of a house, whereas the mortgage is only covering whichever part of the house you don't own. My rent would be $3k, but I own one third of the house, so my mortgage is $2k. The alternative would be paying $3k in rent and investing the $100k in equity. The cash-flow from the equity investment would off-set some of the cost of renting.