Bird Prices

A pet shop has two birds. Both birds were different, the blue one likes to sing and the green one just silent. Then somebody come to buy a bird. He said to the bird seller:
Buyer: How much is this blue bird ...?
Sellers: If you like blue bird that is USD 50 , while the green one had USD 100.
Buyer: Why is the blue bird cheaper than the green silent?
Seller: yah .... clearly different. The green bird is the creator of the song !