Thats a good comparison, cost of machine shop work alloy vs iron. ADD to that the other required bit's and pieces for the alloy block and the price starts to go up considerably beyond an iron block.
As for MYSELF building an original FE block, I would not hesitate to utilize a local machine shop because I know what I want done, how it should be done, what spec's I want. Then I'll do the assembly myself. Granted, by far most folks are going to need an FE specialist, but not necessarily a Gessford or KC level specialist for an original block. For a Shelby block, your options are more limited, you NEED the best, it WILL cost more. Don't forget the shipping costs back and forth....
Resto, I find myself considering that block as well.