[Wintamute]

I doubt they would implement something like this. Their business model is likely built on a) having regular income and b) many users not using all their tasks per month (i.e. paying for something they aren't using). It's the same reason services like Dropbox don't offer a pay-for-what-you-use plan - the majority of their income comes from people paying the set monthly fees but not using anywhere near the maximum storage/bandwidth.

[tych0]

Sure, but there's no reason you can't offer a non-monthly plan at the true operating cost * 1.(your markup). It might be slightly (or vastly) more expensive per request than the monthly plan, but guys like me would still buy 20 or 30, instead of having no revenue from us at all...