The website
www.buyingapuppy.com had a good page on questions to ask a breeder to help you select a reputable one. I believe the YTCA.org site also has similar information. If you are looking to spend less than what good yorkie pups are selling for, you may want to consider a rescue/shelter dog. Their applications are not much more stringent than what you would find from a good breeder and you would be made aware of any problems with the dog in advance. This would be a better option than buying a 'cheap' yorkie that ends up costing you a fortune in vet bills and heartache down the road.
Where are you located?and what were you hoping to spend? There may be someone on this site with just the pup for you.