They cheapest thing ive seen is 10$ shop lights a home depot with a 2700k and a 6500 k bulb and a home made bubbleponics system made from pvc, hose, rubbermaid container, hosing, sprayer heads, etc. Even that system was nice and cheap to build but still prolly cost like 200$ once you include mylar or panda film, timers, fans, pumps etc.
Theres is a DWC systems using a cheap black 5gal bucket and 6 cheap cup thing with 1 air pump 1 airstone and some water and a small thing of superthrive i bet you could make a 6 plant dwc system for 20$ and then just put it outside where light is FREE !
Walmart and the dollar store make most items around a dollar or less only the 5 gal bucket and the air pump are over a dollar really.
do i win ?
lol