A lumber yard near where I live has Black Birch Burl 40" by 40" for $600.00 USD.
They also have much smaller bowl blanks (perfect for scales) of:
Ash
Birch
Bocote
Bubinga
Cherry
Cocobolo
Goncalo
Alves
Genuine Mahogany
Maple. Birdseye
Maple, Soft
Maple, Hard
Maple, Hard Curl
Maple, Soft Curl
Oak, Red
Olivewood
Sapele
Walnut, Black
Typical size of bowl blank and price, though they all vary in both e.g. Cocobolo bowl blank 2" x 6" x 6" are $20.00 USD each