River cruises down the Volga operate during the summer months (early May to end of September). Dozens of boats operated by different companies run from Moscow to Astrakhan. One way or return cruises may be reserved to/from practically any city along the Volga. Turflot.ru and infoflot.ru are several sites that offer tours.
Sample price for cruise: Moscow to Astrakhan (one-way) with three daily meals is RUR 15,000 (approximately 500USD). Without meals, approximately RUR 8-9,000.