The last time I checked, Yorkies were the most popular breed of dog in the US. That's what makes them so expensive. It's the supply and demand thing.
Finding a healthy Yorkie pup for $400 or less would be very suspicious to me. You can probably get some mixed-breed or possibly even unpapered purebreds for that, but please just be careful. Sadly, the Nigerian scams have now filtered out to non-existent dogs for sale because so many people got wise to the e-mail scams about how their great-uncle's third cousin twice removed had left them a fortune just waiting to be claimed. I had a very long conversation with the advertising director of our local paper (Philadelphia Inquirer) about why they were allowing these scams ads, and she said they had no way to screen them. I asked if it was okay if I screen them, and she said yes, and I do that every Sunday. I find the scam ads then report them, and they remove them from the paper.
All dogs are good dogs. Just be careful what you buy