I did it myself. The amp is a Alpine PDX-M12 and that cost me $500. The sub is Alpine Type R 12D4 that cost me $175. Box ran me like $30. So roughly $700.
I used the left and right underseat subwoofer wires for the signal to the amp. I used the power antenna wire for a remote turn on switch. Every thing is right in the trunk so installation was quick, easy, and painless
2013 X3 28i Alpine White/ Black Nevada Leather/premium pkg/convenience/sport/DHP/Cold weather 1/Premium sound/Parking sensors/Alpine PDX-M12 AMP powering one Alpine SWR 12D4