Why guidance changes the cost of a railgun projectile

Would railgun ammunition be astronomically expensive?

Guided ammunition would be expensive because its electronics need protection; a simple unguided projectile is expected to be much cheaper.

A railgun can launch a simple unguided metal projectile. Adding guidance requires electronics that survive the electromagnetic forces of launch and still function afterwards. That protection is the main reason given for expecting an expensive smart round. The exchange provides a design distinction rather than a quoted production price.