Earlier quoted context omitted.
> though they're expensive to run The pens are expensive? Or they use some special paper? Or they wear through some component like belts? What's the expensive bit?
The TCO of the large X-Y pen plotters seemed subjectively expensive, for hobby purposes, which I suspect is most of the reason to still use a pen plotter. Large paper (maybe you can use the current inkjet ones), expensive pens that you go through quickly (I don't know whether they're made anymore, or whether there's a viable DIY way to keep them refilled and working, or to adapt current off-the-shelf non-plotter pens…
But you can throw a pen into any gantry mechanism, whether that's a cheap laser like the k40 or a hobby router like the xcarve, or a kit-built machine like cncrouterparts sells.
These are cheap and ubiquitous. Most of them speak gcode not hpgl, but that's just a firmware tweak or a postprocessor change.