Blue Wilderness Duck is in our rotation. I'm often able to get cheap online only petco gift cards off of ebay which helps to take the sting out of the price. You can usually find a code that will cover shipping (or at least close to the cost of it). The other nice thing about ordering online is that if, for whatever reason, Trixie doesn't like what you choose, you can just return the rest to the store, even if it is only 18 of 24 cases. They'll just calculate the cost per can and refund you for whatever you haven't used. It's $1.59 a can there, which is steep, but if you buy 10 or more, you get 10% off. Occasionally, there will be Petsmart coupons in the Sunday paper, which Petco will also accept.
If you're at Petco, I'd give Nature's Instinct a look too. The food is very, very dense with what seems to be very little water in it. I think that makes it extremely economical... I'd rather pay the cost of water from tap to mix into the food than pay for water as a main ingredient.
Good luck!