Estimate 8.5-8.8 lbs per US gallon of water and mutliply that by 90.
Contact your intended tank's manufacturer to find out the specific dry weight of the exact model of tank you're looking at (AGA should still list the weights in their little catalogue). Acrylic and glass tanks will vary, so be very specific about the tank you're getting to get the estimated dry weight.
Are you going to use sand for the tank or go bare bottom? If you're going to go with sand, estimate AT LEAST 60-75 lbs.
For live rock, I like to estimate based off the high end of live rock. Figuring 1.5 lbs per gallon of live rock at a bare minimum for biological filtration rules of thumb, and you'll have AT LEAST 135 lbs in there. A 90G can hold probably about 180-200 lbs of liverock easily, depending on your tastes.
Add that all up.
So, giving a "ballpark estimate," look to anywhere between 1000lbs to 1300lbs.