Just with a cursory "oil tanker for sale" search, they appear to be for sale in the $10-20MM range.
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.
Details of the above are important to get just right. It works out if you get all the details right.
Barrels 2000000
lease cost / day $350,000.00
Barrel cost -$37
Take delivery profit $74,000,000.00
After 180 days storage $63,000,000.00
Sell/barrel $20
Net $51,000,000.00