Well, for reasons stated I don't believe it was a fair and representative sample then, and that 4% is closer to the upper bound than the lower. Also no idea how "no one was able to give a precise figure" – go into stripe dashboard, open most recent invoice, divide amount paid through payments processed (which are both stated right there on the invoice!)
I'm a B2B SaaS founder and paid 2.7% to stripe last month. OP, feel free to update post with new lower bound.
>We asked this question to dozens of SaaS founders and none of them was able to provide a precise figure. Answers ranged from 4 to 8% of their revenue.