The PCBBot leverages the PCB itself as the chassis. This reduces cost significantly. It also means that the two motors and the caster wheel would have to be mounted on the PCB itself, in addition to the batteries and all the other components...
For more info have a look at the write up on my blog entry
In this Github repo, you'll find my PCBBot KiCAD PCB design freely available (MIT License) along with a Digikey bill of materials (BOM) (caster wheel, motor brackets and motors not included) and MicroPython code. I’m also providing my custom symbol and footprint libraries in this repo. Feel free to make your own version of the PCBBot as is or modify it to suit your needs. Have fun!