For example, I've seen a $100 difference in similar condition HK USPs between the two calibers. Using WWB range ammo as a guide, it would take about 2,500 rounds to make up the difference in price.
I think your math is off.
Lessons cost money. Good ones cost lots. -Tony Beets
WWB 200 pack 9mm is $48 ($0.24 a round) and .40 is $56 ($0.28 a round) at Wal-Mart. So, a $0.04 a round difference times 2,500 rounds is $100. I didn't include tax, but the number should be close.