I know that several people compare the Grand Slam to the Vari-X IIIs. I don't know if that's a fair comparison, but that's what they say.
Anyway, you're right, at $500 it's roughly twice the money of the Grand Slams. Why? I have no idea. In fact, I haven't even seen their tactical model (or their Grand Slam to be honest with you.)
I would assume that the reason for the much higher price, in part, (and again, this is just my assumption) is that it might have a 30mm tube, or special knob adjustments, or special reticles.
I guess that either Chris at SWFA or Jon at the Optic Zone could better explain.
Zachary