Too many factors to consider. how porous the rock is, salinity of the water etc. if you took 65 pounds of dry rock and then made it wet "live rock" obviously when you weighed it it would be heavier being wet. how much heavier isn't something I believe there is a calculator for. it is a downside to purchasing live rock. you pay by the pound and pardon the loose comparison but it's like taking a trailer of trash to the garbage dump. If it's been raining you pay more cause it's wet which makes it heavier, dry garbage = lighter load which = smaller cost. Obviously the "wet" live rock is what keeps it live and what makes it extremely beneficial to us as aquarist's. Simplest way to give you a ball park is just take a pound of dry rock you have kicking around toss it in your tank, take it out when it's all live and give it a weigh. That should give you a rough estimate taking into account some of the variables of course. Hopefully that kind of gives you a direction to go in. interesting topic and I'd be curious if you did the little test to see the results :)