NOYB wrote:

$100,000 mortgage at 5% for 30 years is $536.83 per month.

$100,000 mortgage at 6% for 30 years is $599.55 per month.

Using your numbers:

$536.83 x 360 pmts = $193258

Cost of 5% money in your example is $93,258.

$599.55 x 360 pmts = $215838

Cost of money in your 6% example is $115, 838

Here's an interesting observation: The 20% cost differentiation only applies
when working the numbers from the top down!
It's *greater* when working the numbers from the bottom up.

$93,258 divided by $115,838 equals .80 (so there's the 20% I've been talking
about)

However, expressed as a percentage of increase the number is somehow larger
than 20%! Proof: 93258 x 1.2 = 111,909,
a few grand short of the actual new cost number at $115,838.

(Again, I'm just taking your figures at face value without checking them.)

From that perspective, 6% money can be shown to even *more* than 120% the cost
of 5% money, not less.

The mortgage payment at 6% is 11.683% more than the payment at 5%.

How am I wrong?

You're not "wrong" exactly, you're just using an increase in total payment to
argue that *interest costs* don't increase as much as I have claimed.

There is no number of 11.683% or even 12% that comes anywhere close to
expressing the increased cost of the money when borrowing at 6% vs 5%.