If you are using 1 dvc sub- the Dual 2 ohm model is your only option. You can wire it in paralel at 1ohm. It is not necessarily any cheaper to drive speakers at a lower ohm load; you just have to be sure that your amplifier is capable of handling the lower impedence.
the resistance (ohms) is simple physics...the two subs act as two resisitors in a single circuit.....say you wire the two subs in parallel at 4 ohms each...take 1 over the resistance (1/4) plus the other (1/4) and then take the reciprocal of that...in this case it is equal to 1/4 +1/4 = 1/2, in which case the reciprocal is 2...two subs at 4 ohms in parallel have a 2 ohm resistance...this formula will work for anything, just substitute 1 over whatever the resistance is instead of the 4 that i gave the example with...peace