maxmind already does something like this, although you're just buying credits. twilio does the same thing. Other mail services i've used do credits as well. For my projects/demos/etc - $20/year is where I'm at with twilio (which has led to clients using it and spending hundreds or thousands more, minimum).
With maxmind, $10/year might be all that I use (considering 1000 calls might only be $1 charge).
Ahh - Amazon Services model: charge on a metered basis. You would make almost nothing on small customers, but if some of your customers grow significantly - that's where your real revenue would come in (seamlessly for you and your customers).
With maxmind, $10/year might be all that I use (considering 1000 calls might only be $1 charge).