What type of plastic can be used to produce your own filament?
Filament manufacturers (for example, Colorfabb) also sell pellets, price per kilo would be about 10 times less than the same plastic in filament form.

Out of household garbage only ABS can be easily extruded into filament with Filabot-grade machine. PET AKA plastic bottles looks promising as well.

You can also use certain products "off-label". For example, string trimmer string (line) is simply a nylon filament and can be used as-is in modern diameter-agnostic hot end.