that's pretty hard to answer because there are pretty big differences between a cheaper one piece cast wheel & a much more expensive 3 piece wheel. a 3 piece wheel tire combo can be around the same overall weight as the stock 20's or less, where a heavier cast wheel can be quite a bit heavier.
also when it comes to weight on a rotating assy, weight closer to the outer circumference of the wheel is much more detrimental to performance than weight closer to the center. a heavier cast wheel will not only be heavier, but the bulk of it's weight will be at the outer rim area of the wheel, where a 3 piece forged wheel will have the bulk of it's weight in the center section & have extremely light weight outer rim area, all equaling better performance at accelerating & decelerating the wheel assy.