I'd say that the challenge depends on the business model. Making it efficient enough to be profitable as a free web service will be quite tough, so your estimates are probably appropriate. On the other hand, if they can charge $200/month for it, then those estimates are very conservative...
This is the next step for Google to take. If another company manages get this service out for free first, they will be the new search engine everyone goes to. This alone could be highly profitable if they aren't as reluctant to use ads as Google is.
http://blog.reddit.com/2011/02/ibm-watson-research-team-answers-your.html