I found this interesting. Compare the two images above. You can see that "Once per round" was not on the original card.

I wonder, without the "Once per round" restriction, what would an appropriate cost have been?

An interesting observation. To my knowledge, the only time the "once per round" limitation would make a difference is if you were attacking with Cluster Missiles. They were probably too potent with dice modification from Guidance Chimps on both attacks, so the restriction was added to equalize the benefit across different weapons.

I don't think the card would've cost any more points without the restriction, it just would've favored Cluster Missiles over other potential ordnance cards.

Cluster Missiles most likely.

That's well spotted. I can't imagine it would make a massive difference, probably just FFG pre-empting the question.

Good catch