Sorry about the formatting.
Basically $100,000 conventional mortage at 5.59%, you'll pay at least $69k-$85k in interest. At 3.65, it's $45-$52k.
For every $1 you borrow, you pay back 45 to 85 cents in interest.
If I buy a place for 100k (full mortgage) and sell it for $300k in 21 years, how much did I actually make? A lot of people would think $200k. You have to subtract the interest. So it's only $115k-155k.
The big assumption here is that rates are going to be consistent throughout the 21 years.
1995 - 5 year fixed mortgage - 8.3% without discount.
In the early 80's it was around 12-13% and as high as 23%!
Advantage now: The rates are really, really, really low. For compound interest, the less interest you pay in the beginning, the better because more money goes against the principle.
So in the beginning, try to get the lowest rate possible. 1% of $100k is the same as 10% of $10k. So saving that extra 1% against a high principle is imperative.