patty- that's a really good idea, otherwise it's quite easy to build up just enough tartar and plaque to cause permanent damage to that adult tooth behind it and issues for the gumline. I'd shop around and see if you can't find a vet w/ a more reasonable price that you still trust and have that bad boy removed.
ChestersMom- will your pup loose that baby tooth? Most likely. If she's successfully dropped the rest, you might be in luck. How old is she? If she's over 8 to 10 months, or the adult tooth is fully dropped in behind it and it's not even loose, then she just might be wanting to hold onto it.
I'd certainly have it removed if it's not out by the time you spay her.
Yorkies and most all small breeds are known for not having the best teeth/bone substance and gums in the world, so we've got to do all we can to make sure they keep what little god did give them. Brushing is GREAT, but not always practical to some and so the dog's teeth suffer or they're not big chewers and so on...one ounce of prevention-you know how the saying goes.