A best bet is this Opal Surrounded by Diamonds in 14k Yellow Gold Ring (pictured) for $595 before coupon, $446.25 after. With free shipping, that's $149 off and the lowest total price we could find. This ring features an 8mm by 6mm opal, 0.20-tcw diamonds, and 14-karat yellow gold.
Opal Ring Surrounded by Diamonds in 14kt Yellow Gold. Size 7 is also available at these stores