anywhere from $400 to $800 for two http://www.sonicelectronix.com/item_6396_Kicker+06S10L72+Solo-Baric+L7+Series.html and don't hook up a cheap, over rated, low quality amp like HiFonics to those subs
hifonics is not CEA-compliant which means that their amps have not been tested and don't produce or exceed the advertised RMS wattage with minimum distortion
if you buy a CEA-compliant amp (lower end or top of the line) you will have better results
good choice on subs! Hifonics and Power Acoustik are good brands, you are better off with Kicker or MTX subwoofers.
Crutchfield.com helps a lot when finding out prices!