3D modelling. It's impossible to get into 3D modelling and not get eventually sucked into 3D Printing... Which as other people have explained on the thread, is it's own money sinker.
Just make a friend who is very into 3d printing and you can null out their costs of buying models! There are also makerspaces in some cities that have publicly accessible 3d printers (and other cool stuff like sticker machines, cnc machines, etc) and even some libraries have 3d printers available that you can access