$6.99 a quart at Advance Autoparts!!!
I just got finished with my first oil change at 1620 miles, and asked that they use Nissan Ester Oil, which they did for $59.99. Afterwards I stopped at an Advance Autoparts store to see if they had any DEI radiator adder, they carry the brand, but not any of the fluids. While talking to the guy he said you'd be better off buying a good synthetic oil like Mobil 1 or Royal Purple, and Royal Purple is on sale for 6.99 a qt. I was like "NO SH!T?" So i bought 6 quarts even though I had literally just got done with an oil change. Saving it for my 3750 change.
Check your local stores!