Wikipedia and discussions with another techie friend: http://en.wikipedia.org/wiki/Speech_synthesis#Challenges
90M is a lot; maybe we could work something out? :)
Otherwise, if it's still just dictionary words, it's just a lot easier if the dictionary already breaks down the word into phonemes, and the speech engine then collates the phoneme sounds together. You're not actually using most of the features of a text-to-speech engine, so I don't see why you'd have to pay for the full price tag.
So: how many words? And what's a price that would make you want to buy a license to a simpler engine? :)