In most cases, the cost to transfer something will result in an additional cost of bronze than it is to upgrade it locally in the campaign it’s in.
The only time it’s cheaper to upgrade first, then transfer, is if:
- You’re doing it to a +0 weapon unlocked between levels 1-10 and transferring it over to a campaign where it would otherwise be unlocked at level 11+ (ex. Berlin MG 34 to Tunisia MG 34 / Berlin DP-27 to Moscow DP-27 / Normandy Garand to Tunisia Garand)
- You’re doing it to a +1 weapon unlocked between levels 1-10 and transferring it over to a campaign where it would otherwise be unlocked at level 21+
Transferring something at similar or lower unlock level will result in a net loss.