This picture shows that only 2 padlocks are needed or 16 keys. this makes a padlock have a value of 8 keys.
In this picture, I have sold my Tin. So I need 2 padlocks (16 Keys) and 2 Tin (94-16=78)/2 = 39. Tin has a value of 39 keys.
The next 2 pictures show that Barn Nails have a value of 8 keys each. The first picture shows that I need 10 barn nails or 80 keys to complete the upgrade. (80/10 = 8). In the second picture I need 12 barn nails or 96 keys, again showing barn nails have a value of 8 Keys each.