Clash-of-clans – How is the number of gems required to finish an upgrade calculated

According to this "link"

At town hall 8 ( assumed you just upgraded to level 8 ) :

• canon : 5x (800K+1600K) = 12,000,000
• archer tower : 4x (1500K+2500K) + 1x ( level 1 to 10 ) = 21,368,000
• Mortar : 4x= 6,400,000
• wizard tower : 3x = 16,460,000
• Air defense : 3x = 8,482,500
• Hidden tesla : 3x = 26,250,000
• Elixir storage : 1x =948,550
• Clan castle : 1x = 1,800,000

Total cost for defenses = 95,109,050 golds

walls level 8 : 225x = 128,560,000 - 40x ( already level 8) = 108,560,000

The amount of trophies you earn is not directly relative to the percentage of destruction. It depends on how many stars you earn. ( Read the bold text below )

According to the Clash of Clans wiki, "Raids" article:

For each star that you earn, you receive one-third of the available Trophies. This means that destroying your opponent's Town Hall gives you an automatic victory. Also it is impossible to get more than one star without destroying the Town Hall. Failure to get any stars means a loss, causing you to lose Trophies.

So for example, let's assume available trophy for a battle is 30. If you destroy the Town Hall and 50% of your opponent's base you will receive 20 trophies, and if you keep destroying it until 90%, you still will get 20 trophies.