Welcome to the forums @Liam_Woolley!
You should be able to do this by making {actual cost} into a formula field with an IF()
statement. The formula would look something like this:
IF({cost over-ride}, {cost over-ride}, cost)
This simply checks if {cost over-ride} has a value, and if it does, it uses that value. If not, it uses {cost}.
Make sure to also edit the Formatting on the {actual cost} formula field, you’ll need to change it to Currency.
Hope that helps.