Lucy costs $35 USD, plus the cost of the text-to-speech program (e.g. TextAloud, which I think is <$50): http://www.nextup.com/acapela.html

(Acapela doesn't sell directly to consumers; they license their technology to resellers.)

As a caveat, if you are thinking of buying a voice like Lucy, it does not work with Windows' built-in Text-to-speech (for example - Adobe Reader's text-to-speech feature). It looks like Lernout & Hauspie has some kind of agreement with Microsoft to lock out other competitors.