The simple rule for enhancements is if the part of the card your are enhancing would enhance the card's ability to affect multiple things then it costs double.
If a card was like
Attack 3 o
All Adjacent Enemies
Move 3 o
Enhancing the attack would cost 100g and enhancing the move would cost 30g as the move in no way influences anything related to multiple things being affected.
Beyond that I am reasonably confident that even enhancing a multi-target ability would not double the cost if the end result was the same as if the enhancement was not on a multi-target card. So adding an element infusion to a target 2 attack would not double the cost as there is no way to gain an extra benefit from that.
I don't think the last part of what you said is correct. The move is a separate ability from the attack so I don't think it's doubled, but if any enhancement applied to a multi-target ability should be doubled as per the rule below:
"Double the base cost of an enhancement (other than an attack hex) if it is applied to an ability that targets multiple figures."