I would be in favor of a non-linear formula.
Example:
+ If the wonder was 10% finished, you gain 1% (0.10*0.10) of the prod value in culture.
+ If the wonder was 50% finished, you gain 25% (0.50*0.50) of the prod value in culture.
+ If the wonder was ~99% finished, you gain ~99% of the prod value in culture.
Those values are probably not the good ones to take, but I think you got the idea.