Yeah, vintage builds are... tricky when it comes to expenses lol, but I don't think that they're actually any more expensive than an equivalent build with a new bike... save labor ofc.
Much of what gets replaced/upgraded would be the same items where it a new one being motorized - rims, seat, tires, even hubs tend to be the first things swapped out no matter the age of the bike. What money saved by gettin' a old bike gets used up pretty quick makin' it shine.
I've actually come out ahead on the build I'm currently on ('43 Rollfast). While it's defo gettin' pricey - everything I've purchased for it (other than wire wheel & sandpaper) I woulda had to have gotten anyway, even if I didn't because the new bike included it - it woulda cost more initially.
I suspect that the reason for most vintage builds costing a bit more than yer average is simple - anyone insane enough to go through the process of bringing an old bike back from the dead is prone to also purchasing above average accessories, why get el'cheapo Walmart rims for the thing you just spent so much effort on?
*shrug* mebbe I'm just rationalizing