$500 seems to be a very reasonable price for a healthy Yorkie rescue (with or without papers) - especially if it has been neutered and has had annual vaccinations.  Yorkie ownership is not cheap and the initial investment is the least of the expenses.  I would hate to add up the annual expenses in vet bills, food, treats, toys, etc. for each of my guys.  In fact I purposely WON'T do that 'cause I don't want to know