You were never your in-laws priority and neither were their grandchildren which is what I think rankles you so much.

A million dollar home in LA County doesn't go as far as it used to, and while paying his child support would be like throwing money away, chances are the value of the house will, over time, increase, so from their perspective, it's an investment in their future.

If you can, try to focus on the fact that your children will get to see their father in a stable living situation and you never have to have another holiday meal with your ex-husband's parents.