I had my yorkie's and chihuahua's teeth cleaned last summer when they were a year old. The vet charged me $100 for both. He gives me a discount since I have 2 dogs.
My chi was just done again in March when she was spayed and the vet had to remove a bad tooth! She's 2 years old, eats dry food, and chews bully sticks but isn't great about letting me brush her teeth. She weighs 3 lbs though and her mouth is tiny.
My yorkie will have his teeth cleaned next month. He's does great with teeth brushing but he's 6 lbs and his mouth is a bit bigger.
I'm going to have their teeth cleaned at least once a year because I don't want my babies to have rotten teeth! Have you ever seen a toy dog with missing or rotton teeth? It's so sad!