Well, they should be DVC 4ohm, your RMS will be 500w X 2.....Look for something with these type of specs in an amplifier: 1 or 2 ohm stable 1000w rms (at the most)
1000wrms at the most? give them more. i had my type r's on a kx1200.1 rated at 1400wrms+ with no problems. give each 600-800wrms clean power and you'll be fine.
oh, and those subs are dual 4ohm so lok for an amp that's 1ohm stable. as for cheapest amp, try to look for a hifonics 1606 amp, you'll probably see about 1300wrms from it.