It sounds like the people that contacted you may have been thinking - to breed the yorkie... which is sad. If I were you I would raise the price to $600 for a boy, $7-800 for a girl.... or keep them until they can be spayed/neutered ... charge the person the fee for spaying or neutering.
Be careful to screen the future parents, see if they have biiiig dogs, or rough children.... Good luck.