How can you reduce the cost of one of the products while maintaining its quality and address the changes in the quality of raw materials?

Cost of a product is not completely related with the quality of the product. To reduce a particular cost which  will not impact its quality we have to do backward intergration. 

Help our own vendors who supply us raw materials to bring down their cost of production. This can be done by single sourcing of common materials as it gives a better price. Getting into the cost sheet of vendor and remove unnecessary expenses. Also apply various waste reducing techniques like the5s, Poka YokeTQM etc. This will improve the process and quality at the same time reducing cost.