A young couple I know, recent grads from a state university, share a walk-up apartment in a gritty part of town. They buy their T-shirts used. They share a 22-yearold Honda Accord with well over a century of miles on it. Their health club is weights in a garage. And they spend about $700 a month on food. They know what they spend because they put every single purchase on a spreadsheet. “We spend…