The top number is the amount of gold the Mineshaft yields when the cart gets out of the Mineshaft.

 

The number inside the progress bar is the Mineshaft's level. Spend gold to increase the Mineshaft's level, which increases the Mineshaft's gold profit. When the Mineshaft level is upgraded enough to fill the progress bar, it gets a gold profit multiplier. The gold profit multiplier can be quite big and varies for each Mineshaft.

 

The number with the + sign is the amount of the MIneshaft upgrades you can currently afford. Each Mineshaft upgrade gives a small gold profit boost, so the main focus is to prioritize upgrading the Mineshaft that will reach its next gold profit multiplier.

 

 

Number below is the price of the next upgrade.

 

 

+1 on the gray background means you cannot upgrade this Mine now.