The concept you're looking for is "marginal cost". The initial $50,000 for the machine has already been spent - the only calculation left is that each new widge

The concept you're looking for is "marginal cost". The initial $50,000 for the machine has already been spent - the only calculation left is that each new widget costs 20 cents to make (that's the marginal cost) and generates $2.00 in revenue. At this point, making widgets is highly profitable.