Amazon offers Prime members the Calvin Klein Men's Boxer Briefs 3-Pack in five colors (Black/White/Grey pictured) for $17.59 with free shipping. (Non-members can spend $35 or more to bag free shipping; otherwise, it adds $5.48.) That's $5.86 per pair and the lowest total price we could find for this quantity by $13, outside of the mention below. They're available in sizes S to XL, although not in all size/color combinations.

Macy's offers them for $17.59 with in-store pickup via coupon code "BTS".