“Digicert seems to be our only option now but a certificate there would cost $629 (tax excluded) for a single year.”
Since June 2023 the CA/B Forum requires code signing keys to live on certified hardware, so ImageMagick can no longer sign releases from GitHub Actions. Its sponsored certificate expired and nobody renewed it. Every company that ships ImageMagick inside its products now gets unsigned Windows binaries. None of them would miss $629.