You pay the rate that they quote you. How they arrive at it I'm not sure. It was a different rate than the one I was given by DCL when I booked a cruise by combining points and cash. I believe it was a couple of hundred dollars less than DCL told me but higher than what Dreams Unlimited quoted me. Others I have heard from on this said that they had been quoted a higher price all around. I honestly have never heard of anyone who was told the exact same price by both DVC MS and DCL.
The cash portion is a discounted rack rate and will not go down even if DCL gives other cash discounts. The prices are usually a fair amount higher than the discounted early booking discounts.