At 12% interest and 30 year fixed with 20% down, a $400,000 home would cost the buyer $1,184,961.71 out of pocket. That's $3,291.56/mo. The total interest paid is $864,961.71. Adjusting that to 15 year fixed but everything else being the same, you pay $691,296.80 out of pocket and $371,296.80 is interest. Don't forget the down payments in your calculation, of course.
At 6.576% (the current interest rate), that same 30 year fixed loan comes to $733,909.74 out of pocket, with $413,909.74 being interest. That's $451,051 less than the 12% (this is basic math). That's how much you save.
This does not include insurance or taxes, this is just pure mortgage.