Stephen Wolfram writes what Wolfram|Alpha Pro does and what it will cost you. He says:
Over the two and a half years since we first launched, Wolfram|Alpha has been growing rapidly in content and capabilities. But today’s introduction of Wolfram|Alpha Pro in effect adds a whole new model for interacting with Wolfram|Alpha—and brings all sorts of fundamentally new and remarkable capabilities.
Broadly speaking, this adds capabilities around inputting into and download and customise the output from the system.