I believe the 'value of a dog' is subjective and I don't think it has anything to do with spaying/neutering.
If you're a breeder or want to participate in conformation shows/breeds surveys, etc., then yes, you'll want a dog intact.
(You can do many sports without having the dog intact.)
If I wanted a highly trained SchH or PPD dog, already trained, I wouldn't care whether it's been spayed/neutered.
A trained dog is a trained dog.
Puppies can be a crapshoot...even from working or titled parents.
'Green dogs' are up-and-coming, partially trained, for whatever purpose you're looking for.
With that dog, you'll see it's potential, temperament and possibly structure, better than with a new pup.
Depending on WHY you want a dog and WHAT you plan to do do with the dog, might determine its 'value', for some people.
I've had both, intact and 'speutered' (!?) dogs. I do sports with some and have had others as 'pets'.
Heat cycles in females don't bother me if kept intact.
For me, the spay/neuter thing never been an issue, nor does it determine the dogs 'value', in my eyes.
I love all of my dogs and consider them all valuable.