We never fleshed it out, since that was something we came up with near the time that we stopped playing Struggle.
I would think that as long as you were above your limit, you would pay.
I say that since there may be instances where you are building something that is a two-turn (or more) unit like fighters and you have say a limit of four. You have two on the chart and move them forward, but want to put down three this turn. You would pay the extra point and place three.
On the next turn, the game situation changes and you shift your production to something else. Let's say that you decide that you need to buy something else- so you still move forward the three fighters that are on the chart, but you don't start any new ones (or only one start one new one). In this case, I would say that you wouldn't need to pay the extra PP since you are back below your limit.
Or you could just make some kind of marker that signifies that one of the fighters is an extra one at the time you place it on the chart and you will need to pay extra for it when you chose to move it forward.
Either way, you can have units above your stated limit but you are paying a premium price to get it. The first one is more forgiving since you can come back below your limit and in doing so, pay less for the continuation of the unit. The second version makes you pay the extra PP every turn you move it forward.