If it is strictly about the money, a larger man made diamond can easily cost just as much as a smaller natural one --- while instantly conveying to the untrained eye the appearance of being more expensive.
I suspect there is pressure in the opposite direction these days.
If you see someone wearing a 6-carat diamond, you know it's either a $100,000 natural diamond, or a $4,000 lab-grown diamond. And you'll assume it's the latter, as far more people can afford the latter.
On the other hand, a 1-carat diamond would be more like $2000 natural, $300 lab-grown, making it far more believable that it was natural.
If it is strictly about the money, a larger man made diamond can easily cost just as much as a smaller natural one --- while instantly conveying to the untrained eye the appearance of being more expensive.