The Lucky 38 has base cost of 100 caps which I feel is a little low - especially given how easy it is to make $$$. Would 1500 or so be more balanced or too much?

Honestly I think that might be a bit much just because there's one of each kind of bench about 50 feet from the fast travel location in Goodsprings and a reloading bench about 10 feet from your fast travel location to Gun Runners. It's a convenience thing really, not a serious gameplay advantage. Unless it's a "special" bench, I'd suggest 500 or so unless it's just a prestige item in that case 1500 sounds about right.

If you were to 'earn' through missions, 1500 caps, it would take a long time.
A simple wood workbench with some devices attached should not cost all that much in my opinion.