The first to thing to understand about the difference between a "show quality" puppy and "pet" puppy is that it is entirely in the eye of the breeder.
A breeder must determine the degree to which each pup meets their Breed Standard and select accordingly. Knowing the Chihuahua Breed Standard – and how to interpret it – is how we select our next generations to improve the breed. In addition to health screening, some Breed Standards have disqualifications, which prevent dogs from entering the show ring, such as cryptorchidism (only one descended testicle).
For Chihuahuas, the obvious disqualifiers are merles (it's not a natually occuring Chihuahua gene, so somewhere up the line there's been a whoopsie), size (a bitch might be too big or too small to safely breed from) or bite - a show dog needs a perfect level or scissor bite. A less than perfect bite may not in any way hinder a long and happy life in a pet dog, but it's not going to make a winning show dog and it's irresposnible to breed when you have such an obvious fault which you will then bring through to future generations.
There are many other considerations in the mix and a breeder has to make this call when a pup is still immature. A "late bloomer" might be considered a pet as a puppy, only to blossom into a real stunner as an adult and every breeder who's bred more than one litter has a tale to tell about "the one that got away."
There is an excellent article here by Denise Flaim on the AKC site about Pet v Show quality that goes into much greater detail.
The shocking truth? All dogs are pets. Some just win ribbons every now and then and some have "titles" but that's certainly not a guarantee of quality. Some breeders will go so far as to sell off a stunning, show winning dog as a pet, simply because they are keeping another pup from the same litter and don't want the second pup competing against they dog they are keeping in the ring.
If you are looking for a healthy pet to join your family, whether or not, in the subjective opinion of one breeder it is "show quality" - and there are some wild variations on what that means in every breed! - is a fairly minor consideration and in no way should affect the price of a dog or the expectation of a healthy, happy puppy.
Lastly, Dogs NZ breeders are required to provide papers for their dogs and expliciltiy prohibited from charging for them. And they only cost $40, so don't be fooled by an unscupulous breeder trying to gouge you for hundred of dollars for papers.