Hi and welcome to YT! So glad your little one found you.
The amount for the teeth cleaning sounds about right. Maybe your vet can work out a payment plan with you. Is there maybe a vet school near you? I would make certain though that they are familiar with little dogs.
It sounds as though you are a good yorkie/silky mommy already and are going to give this little guy an amazing life. Good luck and we would love to see pictures of him.