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What you are trying to do is figure out how much energy is being absorbed by trying to spin up the wheels - and it doesn't matter if the wheels are driven or not. The energy required is the same.
The term you are looking for is "Inertia" - the resistance an object has to movement. Normally inertia is proportional to mass - except when we talking about rotational inertia - where it is proportional to the radius of rotation squared times the mass. In wheels, much of the mass is concentrated in the rim area (as opposed to the spyder) - BUT - differences in mass (weight) might not be. Put another way, a lighter wheel might not have a lower rotational inertia since it is possible for the differences in weight to be in the spyder area of the wheel.
And lastly, consider that if a wheel is 4 pounds lighter and the vehicle weighs 4,000 pounds, that's only 0.1% - and considering 4 wheels and double the rotational inertia loss, we're talking about about a 1% change. You may want to consider that when costing these things out.
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