I'm looking mostly at the Arctic convoys shipping raw materials and machinery to Russia, but I guess the question also applies to the Persian Corridor. While there are detailed records about what was shipped to Russia, there's no info on anything or anyone ever being taken back. So would the ships make the return journey empty, or were these ever used to evacuate people, or bring back cargo to, say, the UK?
The Road to Russia: Arctic Convoys 1942 by Bernard Edwards says that most ships returned from Murmansk in ballast, but there were occasions when goods were taken back. The Empire Starlight was to be loaded with timber and ore in April 1942, but was heavily bombed and eventually sunk before it could leave the port.
HMS Edinburgh was also loaded with cargo in Murmansk in April 1942, though slightly more valuable: five tons of gold, Soviet payment for American supplies. Edinburgh was damaged by a U-boat, then hit again by German destroyers, and finally scuttled by HMS Foresight; the gold was [salvaged in 1981] (http://www.hmsedinburgh.co.uk/salvage.php).