Kmart offers the Joe Boxer Men's Thermal Long Underwear Pants in four colors (Coal Mine Heather pictured) for $4.54. Opt for in-store pickup to avoid the $5.75 shipping charge. That's $8 off and the lowest price we could find. They're available in sizes S to XL, although not in all size/color combinations.
Of note, Shop Your Way Rewards members will earn a Kmart credit with this purchase and may be eligible for additional credits and discounts.