My ferret has patches of hair missing and is very itchy...?

alittle over a month ago i took my ferret to the vet because i noticed some hair slightly missing on his what would be i guess "shoulders". She did a skin graph and put him on some alergy medicine thinking that it was probably an alergic reaction to something, it has been about a month now and it is clearly getting worse. My ferret (pepe) is not even 3 years old yet and i have been looking on the internet all night and it seems like adrenal disease seems to be fitting. I am schduling a vet appt for tomorrow ASAP. I work fulltime and so does my fiance but we are on a very tight budget. now my question is....if he does need surgery is it a very serious surgery or do most ferrets make it through very well??? and secondly how much does this usually cost. I will pay monthly if i have to or even borrow money if i must because my ferret means the world to me. the most important question to me is how risky is the surgery? thank you for the answers in advance.