FirstREX offers buyers of high-end homes cash for their down payments in exchange for a percentage of any increase in value. The firm is helping buyers take advantage of low prices as values are poised to rise, says co-CEO Jim Riccitelli. Higher requirements for down payments are part of the demand. "Houses are really affordable right now because interest rates are low, even after the recent run-up, but housing is not that accessible," he said. "If you can't get the house, affordability doesn't really matter."