Looking for a price tag on the massive House climate and energy bill? Good luck. Conflicting financial analyses abound. Some predict the measure will bring financial ruin, others say it will actually save money for most Americans. Economists estimate costs using mathematical models. But in trying to predict the future, they struggle to account for variables -- such as how rapidly technology advances, how fast the economy grows, or how quickly consumers install more efficient water heaters and compact fluorescent light bulbs. "This is not physics, you cannot prove it," said Frank Ackerman, senior economist with the Stockholm Environment Institute at Tufts University. "It's a matter of arguing about what do we think are reasonable assumptions about the future."