I do belive a can of flat white spray paint for 99 cents reflects between 97% to 99% of the light back onto the plants and lasts for about 2 years. Shit you can't get much cheaper than that. Foil is the worst thing you could use as it only reflects 60% of the light back onto the plants and causes a magnifying glass effect onto the plants causing burn holes in the coumpound leaves. If you living in the fucking ghetto and have no choice but to use foil use the dull side atleast.