Yes, the USSR paid a very high price for the defeat of Germany. And yes, the US and UK could have stayed out of it and the USSR would have rolled up to the Atlantic Ocean and we'd be talking about the amazing growth of the Finnish economy since it's independence from the USSR in 1990.
It's unlikely that the USSR would have been able to defeat Germany on its own. In addition to opening a second front in Italy and a third front in France, and sharing military designs to improve Soviet tanks - direct US aid included:
"more than 400,000 vehicles, 14,000 aircraft, 13,000 battle tanks, gasoline and explosives, and thousands of radio sets and motorcycles, as well as food, blankets, machine tools, factory equipment, and boots. The U.S. had provided 55% of the aluminum and 80% of the copper used by the Soviets."
