about £70 per seaker plus amp to run
remove rear speakers, modify cards to accept the extra width, screw in subs, wire up to amp
that will give more mid bass than anything else you fit in the rear
slightly more weight than co-axial speakers due to the extra weight of the magnet
best option for installing these however is to make a selaed enclosure out of fibreglass attached to the back of the interior panel (extra weight again though)