Yes theres a bit more to it than just pistons and a turbo, but if you factors in the original cost of an SR that is the money for a complete bottom end build (pistons, rods, bearing, balance blue print etc), then turbo, manifold, injectors and some sort of tuning method is gonna cost the same for both.
With the KA you would have a fresh built motor with a big turbo, big power etc
With an SR you would still have a used old motors with a big turbo etc.
The difference is that the KA is already there and has 20% more displacment too. However the KA-T always gets a bad name because people boost old ones then crank the boost on turbos which are usually significantly bigger than the T25 that SR’s are equiped with and blow the ring lands. Both motors have the same piston material from factory but the difference in compression between the SR and KA is enough to push the KA to having a higher chance of cracking a piston. And as for weigh…Who cares move your battery to the back of the car or take out the bumper rebar if your so concerned about the extra 50 Lbs the KA weighs.