During my career in retail, I've found buyers don't often ask about flex nibs unless they are 'more advanced' or are following social media where there are many examples for them to see. I caution them about pushing the limits of the nib material; I have seen and repaired many sprung nibs.
On the other hand, I have found that many more customers appreciate the subtle bounce of the semi-flex nib when writing. The difference between a 'semi-flex' and 'flex' nib is affected by many factors so any attempt to quantify it may be quite difficult - it ultimately is the pressure and feedback experienced by the writer that matters.