Water is roughly 8 lbs a gallon, and the shipping weight of a biocube is 56lbs - so that puts us at 312lbs. Sand and rock would add say 25 more lbs (its heavier than water, but offsetting a lot of water).
I'd plan on 350lbs, with it most likely coming up less than that (almost every tank has its gallonage calculated from the dimension of the outside of the glass, and fully filled to the brim. I'd be surprised if a 32 has more than 27 gallons actually running). If you want to be super duper overkill safe, plan for 400lbs.
I did the math out on my Nuvo Fusion 40 at one point and I think its like 33 gallons of actual water space.