As everyone else has said, it depends a lot on the specific unit, especially the undergarments. For me in cool water (over 60° F), the amount of weight I need is comparable to a 7mm wet suit. As the undergarment gets fluffier in cooler water, my need for weight grows. I just got a new suit, and I am finding a need a little more weight than the old one.
I think a lot of people significantly overweight themselves in dry suits. They have to be putting a lot of gas in their suits to balance out the weights I hear people using.