Here is the link about 'teacup' Yorkies on the Yorkshire Terrier Club of America website
http://www.ytca.org/faq.html#A
As you can see there is no such thing as a Teacup, moreover, these small undersized ones are risky and need an incredible amount of time spent with them with the owner being home. Vet bills can be astronomical as it doesn't take much for them to be in jeopardy from any illness or setback.