LAS CRUCES, New Mexico (Reuters) - An extra year of buying rides for astronauts to the International Space Station will cost the United States $450 million -- money that would be better spent speeding development of private space taxis, NASA's deputy administrator said on Thursday.

With the retirement this summer of the space shuttles, the United States is dependent on Russia to fly astronauts to the space station, a $100 billion project by 16 nations that orbits about 225 miles above Earth. Russia charges more than $50 million per person for rides on its Soyuz capsules.

NASA so far has spent $388 million to bolster the development of passenger spaceships, with the goal of having one or more U.S. companies capable of providing flights to the space station by 2016.

The U.S. space agency is backing space taxi development by four firms, including Boeing Co.

The Obama Administration is requesting $850 million for the program for the fiscal year that began Oct 1. Bills pending in the House and Senate cut that to $312 million and $500 million, respectively.

Without full funding in 2012, the United States' ability to stop buying rides from Russia in 2016 is at risk, NASA deputy administrator Lori Garver said at the International Symposium for Personal and Commercial Spaceflight, under way in Las Cruces, New Mexico, this week.

"One additional year of buying this service from the Russians will cost the United States about $450 million," Garver said.

"If you take the Senate ($500 million) mark, take it up $350 million, giving it to U.S. companies today to get to our requested amount, it gives us the best chance to be able to replace this foreign government service by 2016. That's the choice," she said.

Overall, commercial space transportation and related industries generated more than $208 billion of economic activity and more than $53 billion in profits in the United States in 2009, a Federal Aviation Administration report released in January 2011 shows.

"That economic impact is only expected to grow," Garver said.

(Editing by Tom Brown and Philip Barbara)