What is the objective?

Describes how to represent the profit on a monthly basis.

On a monthly basis, the profit can be represented as the sale price per ton (150) multiplied by the amount produced minus the cost of production and storage, like this:

maximize 
  sum( m in Months ) 
    (150 * Produce[m] 
    - sum( p in Products ) 
      Cost[m][p] * Buy[m][p] 
    - 5 * sum( p in Products ) 
      Store[m][p]);