We used a 10" driver, the same cone area in a stamped metal basket with a 2:1 area ratio. The cost of this later piece was only a fraction of the driver - why would it be more? The cost in a driver is in the motor structure, eliminate that and the cost should only be a fraction of the original speaker. Then we used two of these levers. Result; +6 dB more output at far less cost than two drivers with motors. A single lever would have reduced this cost further, but we liked the packaging options of two better.

I think that you calcs have some problems.