TOKYO, Oct 7 (Reuters) - The Nikkei share average dropped to a one-month low on Monday as the lack of progress in resolving the U.S. budget standoff left investors increasingly frustrated, with U.S.-reliant exporters like Toyota Motor Corp coming under pressure. The Nikkei dropped 1.2 percent to 13,853.32 after falling to as low as 13,841.93, the lowest level since Sept. 6. The index fell for a fourth day. The Topix fell 1.4 percent to 1,147.58. Among exporters with high exposure to the U.S. market, Toyota fell 1.5 percent, Sony Corp shed 1.8 percent and Subaru-maker Fuji Heavy Industries Ltd dropped 3.3 percent.