user: Question: How many headphone classes have a US MSRP of $150?
Columns: ['Headphone Model', 'Headphone Class', 'Driver-matched dB', 'Construction', 'Earpads', 'Termination', 'US MSRP']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['iGrado', 'Prestige', '0.1', 'Plastic', 'Comfort Pads', '1/8" (3.5 mm) plug', '$49']
SQL:

assistant: SELECT COUNT("Headphone Class") FROM "Table" WHERE "US MSRP" = '$150';