I forgot to talk about tumor removal, surgeries, and other stuff I forgot to mention.
Adrenal disease surgery is not worth doing. Don't pay $1200+ for an adrenal surgery because the tumors will always come back. And the surgery is hard on the ferrets. The better option is to get the Implants.
Some vets will want you to do annual blood panels(~$200). But it's not required.
And depending on your vet, they may want to do an ultrasound to diagnose Adrenal. But its usually not necessary if the clinical signs are already there: hair loss, females with enlarged vulvas, males suddenly aggressive, etc etc.
My first ferret cost me $1000+ in vet bills. He had adrenal and later insulinoma, but he was also generally a sickly ferret who needed vet treatment constantly. He also got a chordoma growth the size of a marble on his tail that needed to be removed. And that procedure was only $70.
The ferrets I raised from a younger age(currently 4-6) have never had any health issues yet. No ER vet. Just basic vet for vaccines and annual check ups. I do expect some to get adrenal in their elderly years since they are all Marshall Mill ferrets. So there would be the cost of those implants. But I don't think it is totally unreasonable cost wise. Ferrets are expensive pets. But not more than a dog or cat, in my opinion.
Do research. Know the signs of health issues.
Read a lot of ferret forum or facebook page posts. You can learn a lot from more experienced people that are usually more helpful than some vets since finding a ferret knowledgable vet can be difficult.
My first ferret vet was an amazingly knowledable lady. She unfortunately moved. I tried out a new vet and she wasn't good at all and I don't agree with some of her decisions.
I suspected one of my older rescues had stomach ulcers, and she wouldn't prescribe me meds until she confirmed ulcers 100% through surgery. I thought that was unbelievable. I am no way going to put an elderly ferret through such an invasive surgery like that because stomach ulcers isn't a huge deal. Some liquid meds and antibiotics on a 2 week trial would have told me whether or not it was effective or not. And my older vet and several other experienced ferret people agreed with me when I talked to them. Surgery option was totally unnecessary and I was able to get meds from a different vet and guess what? The ferret got 100% better.
My point is find a knowledgable ferret vet. Research about all the health issues so you can make educated decisions whether or not a treatment is necessary. Get second, third, fourth opinions.