Replacing a plenum is almost similar to the installation of a new one. This is because there is not much benefit by repairing a small part of the plenum sheet. The cost of the supply plenum largely depends on the size and the material of the plenum that is being bought. Homeowners should pay attention to the size of plenum that exactly attaches to their furnace. A plenum which is incorrectly sized will not bring about the desired results. However, in case a homeowner is not much confident about the size of the plenum that would be apt for an HVAC system, it is better to get in touch with an HVAC contractor. They will be able to determine the exact size. Generally, it is seen that the cost of plenum is something between $50 and $100 depending on the size and the material. The other cost that one needs to consider while replacing plenums in an HVAC system is the labor cost and the materials cost. It is important to call in an expert HVAC technician to ensure that the plenum is installed properly. However, depending on the company hired the installation charges run between $100 and $200.