Here is a start. Lets look at an ultra simplifed problem. What if the apples don't spoil, storeage is free and the price does not change. How would you calculate your Revenue?
Revenue= Profit - Cost =(Price per Kg)(# of Kg of apples) - 0
Well this won't work for the problem that you have, but it is a start!
Now that we have started, how can we "fix" the above model.
So a first step would be to fix the cost. Then keep "fixing" the formula until it models what you want.
Give this a try and post what you come up with.