Earlier quoted context omitted.
Just with a cursory "oil tanker for sale" search, they appear to be for sale in the $10-20MM range.
If the oil tanker can hold 2,000,000 barrels at -$35/ barrel you are being paid $70 million to store the oil for a few months... Of course, the next question is who pays for transport to/from the tanker, labor, dock fees, employees in ship (unless you can simply "park" it for a fee)
It looks like most tankers are ~200,000 metric tons which at ~8 barrels per metric ton is roughly 1.6 million barrels. You also really only need to store it for a month, I think June futures are around $20 still, and so it's a diff of ~$60/barrel.