It's like paying for minutes using telephone services. Or if you'd pay per minute (or level) in World of Warcraft. A terrible idea in my humble opinion for a development environment.
Except you keep the minutes you've already paid for.
It's not a recurring charge.
You get x amount of time to add events, in addition to the free 40.
At the end of x time you pay for the extra events. No matter what, you own that many events to do what you want, whenever you want, as many times as you like, up until you add more events than what you have paid for in the next payment period.
You would then be billed for those extra events.
If you choose not to pay for them then it works like the free version.
If you want to add any events at that point then you would either pay for the current bill, or remove some events that you have already paid for.
Like paying for minutes, those events would have to be reasonably priced in order to keep people interested.
I would add that the amount of events can be drastically reduced by using plugs.
Plugs that can be shared, or sold to the community.