Our dog who has nasal cancer (which I must add, is very rare) had a scope and it was about $800. They put our dog under to do it. Our dog had a stuffy nose with clear discharge and they did a scope to see if it was a foreign body or cyst that could be removed. If it is a cyst it will have a stalk that they can usually remove it during the endoscope. If it is a foreign body, they can also remove it at the time of the scope. It is apparently common for dogs to breathe things in and they get caught at the back of the sinus area because there is a little lip there. The dog's body then produces a bunch of junk around the foreign body and that is why it wouldn't naturally be easy for it to be expelled from the nose on it's own.
We had a biopsy through the roof of the mouth to determine that our lump was cancer. I should say that biopsies can be wrong. Our dog is doing very well with this nasal cancer so sometimes question if they have made a proper diagnosis. The only way you can really be 100% sure is to have the growth removed and sent to pathology. But, a scope can give you answers and even a solution if it is something they can just snip out.
Whatever the reason for your guy's discharge, I think antibiotics can help because if there is some type of mass, there is sure to be extra mucous and this can be a breeding ground for bacteria. I use antibiotics if my dog sounds really stuffy and it seems to help.
I think the fact that you have a large sized dog that is soon to turn 12 shows that you have given him proper care. I have 2 seniors myself. Senior dogs can look skinny as they lose muscle mass. My 15 year old looks that way but interestingly, she has not lost weight so if you see a rapid loss of weight by the scale, that may indicate something else is going on.
Good luck with everything.