Not sure about that. The major VPNs have similar (not identical) functionality and price. They're competing on the perception that they are more secure than others. In that light, having a passing security audit by a reputable company would be table stakes for a lot of customers. As noted by another commenter already, not having one could also be a deal-breaker on a vendor security assessment by a big company, too.
So this one off audit would have to hit them, say, 6000 extra 1 year subscriptions to pay itself off, let alone turn profit. Sounds like a stretch.
It would certainly be hard to measure.
On that note, how many customers do these public customer oriented VPNs have, typically? 1k's? 10k's? More?