ATLANTA — A new study suggests the growing share of income earned by the rich could raise the costs of a Republican proposal to replace Georgia’s income tax with an expanded sales tax. The new study by S&P shows that income gains to the top 1 percent of earners come at a larger cost to the rest of earners. Not only does rising inequality appear to stunt overall economic growth, but S&P links it to a slowdown in average yearly growth in state tax collections.