I think the psychology makes sense. When they told us they wanted to pay on a pay per use basis they based that completely on the past. They did the basic math. Last month I did 16 tickets. At $.25 a piece I would have paid $4! That's what I want to pay!
When faced with a future purchase decision, however, the mindset immediately changed. Now the decision was unbound. What if in three months I get to the point I have 1,000 tickets?!??! My god, now it's $250/month. That's crazy expensive! That fear caused a lot of issues.
So we tried the logical next step. $.25 per use, capped at a certain dollar amount ($25/month). That was too complicated to understand.
So we just started charging a flat $25/month (sales-leads fwiw). That worked really well.
My guess is that is what gives.
To add to this, even when we had implemented this for users, they were our worst customer by far.
Plus a chart that shows you significantly cheaper than three competitors is always a good incentive for some people to get the credit card out.