Gabby Johnson wrote:

Clemens invested $300,000 (including much of his wife's inheritance) in the Paige Compositor, which lost out to Merganthaler Linotype in the mechanized typesetting race. That's about $6 million in today's money.

That might be the source of the $200,000 figure, but that's an investment in typesetting and not the price of a typewriter! So I stick to my guns there. 

He had to go out on the lecture circuit in his old age to make up, partially, for the loss. 

This reminds me of Grant writing his memoirs while dying of cancer to provide for his family - I believe Grant also suffered investment reverses. For this however we can be thankful for a great set of memoirs, whatever his motivation to write them.