Charging developers to try out your API. Questionable. If anything, don't they want to do everything they possibly can to attract developers?
Looking at their visitor stats (http://siteanalytics.compete.com/wolframalpha.com/) I'd guess they have unused capacity for at least 2MM queries a month if back in May each visitor was hitting it twice. I don't get the impression they are using an infrastructure that they only pay for on demand.
If they want anyone to use their API (or their service as a whole) they should give away at least 1000 queries a month to the first 2000 developers that sign up.