The cheapest type of ductwork is typically made of flexible plastic material such as polyethylene or PVC. These materials are relatively inexpensive compared to metal ductwork, making them a popular choice for residential and light commercial applications. However, it's important to note that flexible plastic ductwork may not be as durable as metal and may require more frequent maintenance or replacement. Additionally, it may not be suitable for certain types of HVAC systems or building codes, so it's important to consult with a professional before choosing ductwork material.