Demodex - Zelda had it. Our vet went the dip route. The price you were quoted seems quite reasonable. Ivermectin works too. Going through puberty will often bring it on. That's a big stress on the dog and that contributes to bringing it on a lot. That's probably why the vet mentioned getting him fixed.
The first thing you have to do if ind out if it's considered just a spot or two or if it's generalized. If it's just a spot many folks will wait and see igf it clears up by itself - it often will If it's considered generalized, it needs to be treated.
I can not stress this enough - RESPONSIBLE BREEDERS DO NOT BREED DOGS WHO HAVE HAD GENERALIZED DEMODEX!! The weak immune system that allows it to flair and generalize is an inherited characteristic. If you breed a dog that has had Generalized Demo you create more dogs that will have Generalized Demo. If we ever want to have EBs considered anything resembling healthy we've got to stop breeding and accepting unhealthy one. "It's just one of those things EBs have" will never go away if we don't make it go away. End of sermon.