Hi Mogi - I feel its a trap to make cost decisions too early. You have to make the "best" decision. Sure steel is cheap but you will encounter lots of issues designing with it. You can halve the volume of things by using in-fills/honeycombing voiding. Even add styrofoam bubbles to the mix to halve the density.. Even add air to the mix to make lightweight concrete. But its up to you... Keep researching. Peter