20-0 DEGREES
Under Armour UA Evo ColdGear® Fitted Legging
Click Here to Buy It Now at Under Armour, $50
Don't be thrown off by the price, these are worth every dime to fend off the winter chill and you'll pull 'em out every year.
40-20 DEGREES
Hanes Thermal Pants
Click Here to Buy It Now at Hanes, $6
This is THE CLASSIC long-john. No need to reinvent the wheel.