Can I update my old computer to Windows 11 and how much it cost?

You can try the product server trick to update an old computer to Windows 11 and it costs zero.

The Product Server method is a workaround that allows some unsupported PCs to upgrade to Windows 11 by modifying the upgrade process. Instead of performing a clean installation, this approach uses the Windows setup command with a special product server parameter to bypass certain hardware checks while keeping existing files, apps, and settings. It is mainly useful for older computers that fail Windows 11 compatibility checks because of unsupported CPUs or missing requirements.

setup.exe /product server

Choose Keep personal files and apps when prompted, then continue with the installation process.

The computer will restart several times. After completion, check Windows Settings to confirm that Windows 11 is installed.