Budget! ? I have a wnr3500L with USB & if u need ready made solution I can add a 500 gb portable USB hdd
http://www.smcinternational.in/index.php?page=shop.product_details&flypage=flypage_images.tpl&product_id=2274&category_id=120&option=com_virtuemart&Itemid=465
http://www.flipkart.com/seagate-staa500305-2-5-inch-500-gb-external-hard-disk/p/itmdbm78gcdqsjzz?pid=ACCDYTB2QC62YQ8D&ref=4dbb9d0b-fa80-41a2-9e70-68a42bb63d22&srno=m_1_2&otracker=from-search