It's pretty easy to work out how much a cage should weigh by measuring the linear metres of tube, the OD and the wall thickness (can measure with ultrasonic thingy) or ID and working then out the volume of steel. Then multiply the volume by the density of steel. The density of chrome moly is almost exactly the same as mild steel. Add a couple of percent for gussets, tabs, footings and filler wire/rod.
40kg is in the ball park.