For less than that you could buy 50lbs of a basic chick feed or layer feed which unlike rice would provide everything they need. There are ways of cutting down on feed cost such as free ranging, fermenting feed and feeding them only a bit more than you know they'll eat every day (or using a treadle feeder which would be better) but feeding them only rice isn't it. They are very likely to have nutritional issues, especially if they can't free range and that's going to more than offset whatever you may have saved when you now have to buy poultry vitamins and/or raise replacement birds if any die from nutritional deficiencies (and you will still have to switch to a proper commercial feed). As tempting as it seems, don't do it, your birds will suffer for it and so will your wallet ultimately. Even the cheapest commercial feed is better than rice