What is the best available software for natural sounding text to
speech?  We are willing to pay any price.

They are all rubbish.

Save your money.

I would say that Ray Kurzweil's products, specifically made to help
the blind, are the best on the market.

http://www.kurzweiledu.com/press_070606.asp
-- this is a portable reader that will convert any text, such as
restaurant menus, to speech.

The Kurzweil 3000 is the device most commonly used.
http://www.sightandsound.co.uk/pages/Kurzweil_3000.htm
http://www.tcnj.edu/~library/about/accessibility/kurzweil3000/#prep

There are many websites about text readers for the blind, and probably
somewhere that evaluates them.  Not to question Keystroke's
recommendation.

I could imagine that programs for the blind may not be the most
natural sounding readers, there probably being a trade-off on price,
convenience of use and speed with accepting lower speech quality.

I am using the Community Edition, (i.e. - free version) of Mandriva
Linux LE 2005, and to my surprise one day I was looking in the
Configuration pages, and there was a Text to speech item. I discovered
if I pointed it at one of my Gutenberg books, it would happily start
reading it to me. It's not perfect, acts like alpha software, but it
was legible except when it hit certain Gutenberg legal material, when
it obediently read things like dash asterisk asterisk dash parenthesis
asterisk.

It was right, just not especially relevant.

The voice was quite legible, sounded a bit clipped on some words, but
generally legible.

If this can work this good, and comes free with Linux, I would
certainly agree you should be able to get better for money.