Hi welcome to YT! I doubt that you are going to find a great quality HEALTHY yorkie for 300 dollars. I was just like you a year ago..I really wanted a puppy and but didn't have enough money to spend on one. I searched the web looking for yorkies at a lower price. What I found is that if someone is willing to give up a yorkie puppy for under 500 $ then there is some sort of catch.
So what I did was slow down and reasearch the breed and breeders! Especially through this website I was given a lot of info about the breed.
So here I am a year later and I'm about to get my little girl from a breeder I found on this site (JeanieK) someone I know takes pride in her yorkies. While I was doing my research I was able to save up money, as well as make an informed decision.
So my advice is to take it slow research and while you are doing that you'll be able to save more money to get yourself a healthy baby!