An RSJ (Rolled Steel Joist), also known as a steel beam, can vary in cost depending on its size and length. On average, a 3-meter long RSJ can cost around £100 to £150, while a 6-meter long RSJ can cost between £200 to £300. However, larger and longer RSJs can cost significantly more. It's best to consult with a supplier or contractor for an accurate quote based on the specific requirements of the project.