Vintage pen plotters (or X-Y recorders) are essentially zero-price items and tend to be built like actual tanks. The ones you can buy today (and afford) are way more expensive than them, yet cheaper (in quality). However, vintage pen plotters tend to not have USB plugs.
I was confused too, so looked up the prices. The pen plotter seems to be in the neighborhood of $225 USD [1] while the AxiDraw is $475 USD [2].