Happiness is a very individual thing, and is inherently unreasonable, as it cannot always be reasoned out. A reasonable amount may not be the best way to put it, but an amount that brings about happiness at a greater rate per dollar than other alternatives would be the most logical decision to put money intended for enjoyment on. I see where you're coming from, as the lottery is a terrible investment, but a couple dollars for a chance to dream may be a small price to pay for many.