Unless it's a per/something charge the numbers wouldn't be significant. I would guess there's less than a thousand serious API developers.
I've heard numbers along the lines of millions of Twitter apps. From last year: http://mashable.com/2011/07/11/twitter-1-million-application... Who knows the percentage of serious apps. Twitter probably. Still, I'd imagine even would-be serious apps would pay for minimal Twitter API access.
addendum: This was for 2 actual production applications.