It sounds to me like you understand just fine--amps that are stable at 2 ohms mono are expensive because they draw a ton of power and need large power regulators (we're talking 4 gauge power connections). What I did on my last system was to buy a normal 2-channel amp and then wired one channel to each voice coil--that maintained the 4 ohm load and provided enough sound for me.
Just because a sub is rated for 200/400 doesn't mean that's the minimum you need to run it. Volume is a logarithmic curve
, so you actually have to double the power in wattage to be able to hear much of a difference in volume (3 db increase).
That last 12" sub that I had run (Punch DVC) was at 160 watts and it was loud enough for me--it all depends upon whether or not your are running a sealed box, vented, or bandpass and how much volume you expect. If you just want thump then buy a bandpass box and even a reasonably priced amp will pound. The box that you choose will make a bigger difference than 100 watts of power will.