I do the exact same thing the guy in the article does (but on a much smaller scale), so I'll give you some exact numbers.

I bought about a hundred of these on clearance at Wal-Mart a few months ago.

https://www.amazon.com/dp/B01D67PGLQ

I paid 50 cents each for them, and here's the profit breakdown.

Sales price: $6.42 (prices fluctuate, this is the average price I sold them all for.)

Cost: -$0.55 (50 cents plus 10% sales tax)

Amazon Fees: -$4.13

Shipping: -$0.03

Profit: $1.71 each