Most of the custom pen makers I've tried to contact lately had multiple year wait lists. A few years ago it seemed like you could get one made in a a few months. US makers charge $200-500 for fairly standard custom pens and for more specialized stuff the prices can get even higher. The wait lists are still long. Personally as a person with a collection of vintage pens worth several thousand dollars, I wouldn't pay much more than $500 for a new custom fountain pen unless it had a lot of gold or silver hardware / overlay.
I have a couple of custom pens. One was about $500, used a rare celluloid material no longer in production, and had sterling silver hardware and an 18k nib. The other one was just as well crafted, but only cost about $225 because it was from someone relatively new to the market and it had steel hardware.
I'm sure someone would pay more if the prices went up significantly, but I would either just buy a vintage pen that writes better anyways or I would stop collecting.
Also, so far there doesn't seem to be a secondary market for scalping people with custom pens.
I dabble in woodworking/woodcarving too though, and there's a lot of people flipping custom carving knives for 3-6x what the makers charge. Its a pretty obnoxious practice.