Turbos are not simple, lots of plumbing of intake and exhaust, any free space under the hood is gone, lots of heat under the hood, got to have the right blow off valves, ignition timing becomes a lot more critical... lots more complicated. You have to have some kind of head turbo or not, econo W2s are very reasonable, got to have some kind of crank weather turbo or not and 4.25 is very reasonable for a big block or small block, you got to have rods and pistons weather it is turbo or not, a nice stroker can be had for either one very easily and if you are building a bottom end to handle 15lbs of boost it will not cost any more to build a big stroker with out the added complexity up top.

As for the weight differance in BB and SB I was stateing the weight differance I personally measured between a 67 440/727 and a 71 340/904 (I know a 340 never came with a 904 but the one from a 318 bolts right up ) both still had the iron intakes iron heads... and the differance between the two was almost exactly #200 a low deck might be a little closer but I did not have one to weigh.