I guess your vet is suggesting using the DDAVP (Desmopressin) inhaler as eyedrops. From what I can tell, this stuff runs about $78 a bottle (less if you get more than one bottle).
I am kind of confused as to why you are being prescribed DAVPP for nephrogenic diabetes insipidus, because that's usually prescribed for CENTRAL Diabetes Insipidus, caused when the pituitary gland doesn't secrete enough ADH. NEPHROGENIC Diabetes Insipidus is caused when the kidneys DON'T RESPOND to the ADH being secreted by the pituitary gland.
DAVPP is a drug that mimics the actions of ADH. If the DI that your dog has is nephrogenic, it won't help for the pituitary gland to produce MORE ADH because the kidneys don't know what to do with it anyway. For nephrogenic DI, there is a drug called chlorothiazide. It's an oral drug that causes the kidneys to concentrate the urine so that it's not just like a case of water in/water out.
There IS always the option of NO treatment, but if you opt to do this, there MUST be water available at ALL times, NEVER letting it get to the point where the bowl is empty for even an hour. If you think you can be extremely vigilant, then this might be something you could at least TRY until you see if it's going to be enough to maintain your dog. He'll drink for literally half an hour sometimes with DI and he has to have enough water available to be able to do this.
Just be sure to make your vet aware (I know, it sounds stupid but many vets aren't aware) that your dog is a COLLIE before he goes putting drugs into him willy-nilly. Probably 85-90% of the collies in this country have the MDR1 gene mutation which allows drugs to cross the blood-brain barrier and become toxic to them. Non-collie breeds do not have this problem. This is why it's so gawdawful if a Collie gets heartworm. They cannot safely be treated with ivermectin.
Please keep us up to date on how your collie is doing. I ADORE collies, they are my breed of choice and I just got a puppy a month or so ago after I lost my sable and white female, Katie, last May. I'd love to see pics!
Ghilly