For a value, look at eBay or other sources that give you prices.  For a
17" Stella, I would guess in the neighborhood of $7000, depending on a
lot of other things.

The problem that you have described in the mainspring sounds like a
broken end, and in this case the tip that connects to the spring barrel.
As problems go, it's a relatively painless one to repair, and once it's
repaired, it doesn't usually come back for the next 100 years.  The
repair causes no noticeable decrease in performance, and is quite simple
for someone that has the correct skills and tools.

But it should not be attempted by anyone unfamiliar with large springs.
The Stella mainspring will have around forty feet of spring compressed
into a 5-inch circle.  The natural tendency of the spring is to want to
escape those confines -- and it has a lot of power in it.

I dealt with a Regina that had a broken mainspring for a customer once.
I found that a new mainspring cost around $200, as a ballpark estimate.
His mainspring was broken about eight feet from the end.  I told him his
options, which were for me to install the spring eight feet shorter
(decreasing the total power) and try it out, or for him to buy a new
spring.  He decided to try it shorter.  After I greased and re-installed
the spring, he announced that the shorter spring plays longer than his
other Regina does, with no apparent effect on the power.  While I
believe that he is exaggerating, the truth is that there wasn't much of
a difference afterwards.

Eli Shahar