Gold cost of building upgrade

[Stevekov]

Has anyone worked out the optimum point to use gold to complete building upgrades. What is the reduction rate/hour ?

[Chimera]

In my experience, building upgrades (not land expansions which are less gold per minute), 1 gold is equivalent to about 12 minutes of remaining upgrade time. It varies fractionally.

[Juggernaut]

In my experience, building upgrades (not land expansions which are less gold per minute), 1 gold is equivalent to about 12 minutes of remaining upgrade time. It varies fractionally.

What he said plus

Last 25min =1 gold.

So 1 hr left = 4 gold.

Rest every 1 hr left add 5 gold.