Audacity is an amazing audio editing program. It lets you export any format as well. Its completely free. You will need to make an additional download though if you want to export MP3 format. Its all completely 100% free.
EDIT: My bad I misread your request :p
Because the text is converting to a database of audio of samples, you will never really get anything sounding completely realistic. Definitely not for free. I've heard a few that have come close but still have that very synthetic sound to it.
i7 -2600 CPU @ 3.40GHz - Windows 7 - 8GB RAM - Nivida GeForce 420 GT