> "Ask yourself 'would I buy this today?'"

How would I buy a notebook filled with my thoughts from 20 years ago, today?

It's phrased "would you." I would definitely buy a notebook filled with my thoughts from 20 years ago today.

If someone kidnapped it and you where asked to pay 400$ to get it back. Would you pay the ransom?

I agree. Besides, one is really not 'buying' - just holding on, with no immediate 'expense'.