I have "built" up 2 or 3 lugers over the last years and I don't think I ever 'get" my money out of them when I eventually sell them (something I want always comes up).
So, point is, a built up luger is "cheaper" in the short run, but "much" more expensive in the long run, as individual parts easily add up to a lot more; plus they may require some "fitting" to get it to work right, although my shooters have always functioned fine
Ed