depends on the foam used,but if you use one pound/cubic foot eps foam[virgin] it is the white foam.typically they built the same weight as a built up wing but are alot stronger.as far as cutting your own ,it can be done but for the cost of shipping the blank foam to you and the equipment it is easier just to buy the core allreadycut.I believe there are miss norway cores available.try here
http://www.dbalsa.com/foam_wings/alphabetical.htm