If you use your garden bed to grow herbs, veggies or annuals you probably only need 3-400mm of soil.

Don't fill the bed to the brim. Leave about 100-120mm of space for mulch to retain soil moisture.

To provide good drainage for the bed and save on the amount of fill you need gather up the stones and broken pots and bricks lying about the yard and place them in the bottom of the bed. Break up the Styrofoam you didn't want to put into the general rubbish collection, and add that to the bed. The layer of porous drainage material can be as deep as 150-200mm.