The cheapest I've been able to find for genuine GI 50 cal cans is at Midway USA. I bought a couple of the grade III cans for $7.98 each. They are regular $9.99, but that was my birthday price.
They were actually more like grade I condition. No dings or dents. Tiny bit of surface rust.
Not sure, but I think the ones Costco and Academy sell are made in China.