About: The first 10,000 requests per month are free. Thereafter, requests are charged at a rate of $.00015 each (just 15 cents per thousand requests.).
http://pages.alexa.com/prod_serv/WebInfoService.html
Randy: Alexa is offering their data for a price, a really small price. This is very monetizable. Imagine, I construct a page with 3 Alexa Web services calls. The cost is $0.45 CPM. Throw some AdSense on there and surely you can get between $1-10 CPM.