A SUB Woofer cannot function without an amplifer. So you will have to buy an Amplifier for sure. On top of that you will also have to buy an enclosure for the SUB woofer and also a wiring kit.
Even if you buy all the basic stuff, it will cost you slightly more than 10k.
Sony 2-ch AMP~3.5k
Sony SUB-woofer~5k
Wiring~1.2k
Enclosure~800