Simple coil switching of the motor coils is definitely the simplest and cheapest solution. It reduces losses in the drive circuitry and it also gives you more power out of the motor compared to a sinusoidal drive. The cost is reduced motor efficiency, increased motor heating, and increased noise. It's certainly possible to design a sinusoidal drive that will undervolt well, but cost, efficiency, and drive circuit dissipation tradeoffs may weigh against it. A sinusoidal drive shouldn't add more than $1-$2 to cost of manufacture.
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