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Geoffrey, you wrote:
"Yes, a ratio of two to one on chronometer to good sextant price is my reckoning, too "

I did a little more poking around today on Google Books, and there are some 
good tables of prices. That forty pound price c.1850 for chronometers was 
what the government was paying for chronometers for the Royal Navy. To me 
that suggests that were good, though not excellent ones, were available at 
lower prices. Of course, I should add that a "good sextant for lunars" and a 
"chronometer" are not commodities. They don't have to be equal in price to 
tip the scales, but when the prices got so close, the scales were past the 
tipping point.

You wrote:
"Chronometers tended to stay with the ship. Chronometers were not like cars 
today, where after five years the thing is out of fashion, clapped out, and 
you trade it in for a new one. There were chronometers in the British navy 
that stayed in use for over a hundred years!"

There's an article in the "Nautical Magazine" from 1858 entitled "Chronometer 
Routine" available on Google Books here:
http://books.google.com/books?id=9IQEAAAAQAAJ&pg=PA349
which has some good info on this topic. In fact, chronometers were retired 
from service and returned to their manufacturers for re-cycling on a regular 
basis. In the table in that article, about 42 chronometers were purchased new 
for the RN in 1850-1857, 10 were retired/recycled, and hundreds (nearly equal 
to the total number in service) required repair. I have no doubt that there 
were some chronometers with the amazing longevity you describe, but I believe 
this table, too.

You wrote:
"If the two chronometers disagreed, you are back to using lunars to check which is in error."

When we talked about this two months ago, Gary made a good point. If you have 
two chronometers, mostly they would disagree by some small random amount not 
worth noticing (but you could average if you cared to). But if one suddenly 
changes its rate, which seems to have been how their performance degraded 
back then, then you would either take action at the earliest opportunity to 
get another check on longitude (lunars if you can, speak another ship if 
you're in busy waters, hunt down a small island with a known longitude) or 
you would simply work under the assumption that either longitude could be 
correct and plan your course accordingly. They're not both wrong (highly 
unlikely at least); one or the other longitude is nearly correct. So if you 
can rule out one of them (e.g. "from here we should see the mountains of the 
Galapagos if chro. #110 is right, but the horizon is clear") then you can be 
very confident that the other chronometer is functioning correctly. It's a 
question of failure odds. Of course, with three or more chronometers, you're 
in much better shape.

-FER



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