adidas via eBay offers its adidas Men's Techfit Fitted Training Shirt in Black for $15 with free shipping. That's the lowest total price we could find by $18. It's available in sizes S to XXL.

Update: Of note, buy two shirts for $24 via coupon code "C20HELLOSPRING" when you check out with PayPal. With free shipping, that's $12 per shirt. Coupon ends March 11 and can only be used once per account.