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There are many calculations for electromagnets which depend on the number of coils and the tightness of the coils. And, the amount of resistance depends on the wire size. Copper is an excellent conductor and there is no need for more exotics (except in the superconductors, but then you also have to account for cooling. Most NMR & MRI magnets are supercooled.However, your power consumption should be more or less directly related to your magnetic field. One of the variables is the distance between the field and your material which needs to be minimized.What I would do is at least use the core being a very big rare earth magnet.The most common are Neodymium (Nd2Fe14B) magnets. Then you just need to figure out how to break the magnetic field. There is a type of magnetic base stand that you can buy for laboratories that are really cool. With virtually no effort, you can shift from a relatively strong magnet to virtually not magnet. Here is a description on Wikipedia.http://en.wikipedia.org/wiki/Magnetic_baseI think there are other methods to break magnetic fields too. Airport sweepers often use permanent magnets and have a way to release the debris.
Hey! No fair. The OP said "electromagnet" 
I am curious about the watts per pound, lift.
"I am curious about the watts per pound, lift. "Zerohttp://en.wikipedia.org/wiki/Superconducting_magnet
Anyway, from the definition of the amp we can show that the force is proportional to the current but independent of the voltage. If you get the resistance arbitrarily low by using really thick wires then you can get as close to zero Watts per pound as you like. It's just that the magnet might be rather silly.
I think that if you want the magnet to lift scrap you should use iron with a fat hysteresis curve so that that it has a high degree of magnetism after the power is switched off.Of course to dump the load you would need a short pulse of reverse current to demagnetize it but I think as more time will be spent getting it into position than actually carrying a load the overall power consumption will be reduced.
when push comes to shove, resistivity is king.