Personally I will only buy pants 20K or higher, although 15K+ is probably sufficient. I definitely wouldn't go any lower than that unless you plan to buy new pants every season, use waterproofing wash/spray regularly, and don't do much hiking/sitting/falling.
15 NS Snowtrooper Brother in Arms Edition . 13 Salomon Pow Snake
15 Now Drive . 14 Now Drive
14 Flow Hylite