I have a pair of DIY Sonosub Subwoofers that I built about 7 or 8 years back.
 
Each subwoofers contains a 15" TC-Sounds TC-2000 4ohm SVC subwoofer. These are passive subwoofers - you will need your own amp.
 
I'm going off memory, but I believe they are 320liters of internal volume and are tuned to about 14Hz.
 
I paid nearly $350+shipping for each subwoofer at the time. I also paid a $100+ for the 24" sonotube.
 
I seen people sell old SVS passive subs for $300.
 
Don't know what is a fair price, but I was thinking $250/each of  $450pair.