It's not flip flopping at all. Again, "half the mortgage" is not fair or equitable if she isn't getting half of the equity, and "half of nothing" is not fair if you own the home outright. In the wholly owned home scenario, half should be "half the fair market rental value for an equivalent space", like I said above. Whether that's more or less than what half the mortgage would be is irrelevant. And whether a mortgage would be more or less than rent is dependent on a whole slew of factors, like amortization period, equity, and so on. If you have a 30 year mortgage, yeah it might well be cheaper than rent. If it's 15 years, no way it will be. And again, that's really going to be influenced by the local real estate market. Throughout most of Canada it's very much a renter's market right now because we never had a real estate correction, so home values are way higher than rents. The US is much more balanced, but certain regional markets may not be.