The concept was tried over seven years in a much smaller prototype in Spain. But to be truly efficient, its designers say, scale is of the essence. A 3,000-foot tower wouldn't come cheap: The giant structure will cost an estimated $670 million. The company argues that an expected life span of 100 years, combined with low maintenance costs and the absence of pollution, will make the endeavor worthwhile. The Australian government is expected to OK the construction, but so far has offered no money.