EIP-1344: Add chain id opcode

I agree, it is not as simple as “Is it expensive or not”

For me it is a combination of, how much gas would be appropriate for such a call and how often would such a call be used.

Probably an argumentation for this should be added to the EIP. Something like:

Signatures are widely used (e.g. state channels, multi signature wallets and relays) currently all these signatures can not be properly protected against cross-chain replay attacks. As we assume that all these signatures will use this opcode it would be preferred to use an opcode, to be gas efficient.

This might be something else if EIP-1109: Remove call costs for precompiled contracts would be part of the next hardfork too