I've got my first Adrenal ferret scheduled for surgery on Friday.  My
regular vet is going to charge $350 if left, $550 if right.  I imagine
biopsies will be an additional charge.  I called the ferret's old vet
just out of curiosity, and he quoted $225 for the surgery, but he's had
less experience and never removed a right tumor.
 
I recommend you read the Adrenal FAQ, which will list all the possible
risks.  For example, studies show a 10-12% rate of post-operative
mortality, presumably because ferrets can't compensate for the loss of
hormones.  It also lists chances of left being affected (65%?  something
like that) or right (26%?) or both (about 10%).
 
It is here:
http://www.ferretcentral.org/faq/med/adrenal.html
(I printed it to discuss with my vet.)
 
This is a constantly updated survey which will give you an idea how often
the surgery fails and things like that:
http://www.miamiferret.org/FHC/vote.htm
[Posted in FML issue 3868]