Mostly the "pocket" and "micro" bullies are poorly bred dogs, anyway, so I wouldn't worry too much about it, but the reason they can charge so much is because they're marketed as a cute "designer dog" and because people will pay for cute designer dogs. It's a scam, really, and likely a puppy mill. They likely keep the price a secret because they want people to continue perusing their photos and fall in love so they'll be more likely to pay those outrageous prices.
Actual reputable breeders are not always upfront with their prices, either, because it depends on many things from if the bitch needed special vet care or if she was artificially inseminated, etc. Some might not tell you until you've filled out an application, and others have it posted on their websites, so it depends. I would expect a reputable breeder who health tests and titles their breeding stock to cost around $1,500 to $3,000 for a pet. You're paying for the time the breeder has spent meticulously researching each parents' pedigree, genetic and OFA health testing, and the time she's spent proving her breeding stock, but you're getting a very well bred dog.
It looks like the American Bulldog Association is the main "club" for the breed, but they don't support the AKC recognizing the breed? Maybe someone more familiar with this can chime in and explain, but this is probably a good place to start looking:
American Bulldog Association
This website will show you what health tests reputable breeders should be doing, and perhaps if you send them a message they would provide a list of reputable breeders. Some other clubs have them on their website, but this one doesn't appear to. Again, I'm not super familiar with the breed!