Gamersgate has a coupon SOW18 for an additional 18% off Shadow of War. Looks like historic lows aside from "y u no get glitch" pricing or video card promo offers.
Base Edition - $29.51 ($25.09 zGold)
Silver Edition - $39.35 ($33.45 zGold)
Gold Edition - $57.39 ($45.91 zGold)
Sadly, none of those work out clean for buying zGold unless you already owned some. You'd have to buy $30 worth for base, $40 for Silver and $50 for Gold. Still, $50 for SoW Gold versus $57.39 makes it worth doing even if you'll be buying more than you actually need. You'll currently get $57.50 worth of zGold so you can throw another ~$14 game in there for "free" after discounts.
Edit: I said you get $55 for $50, it's actually $57.50