Summit has AC Delco AGM batteries for $142-162 depending on case size. Most are 800-900 CA. I don't know where they are made or if they have a decent rep.
Long thread on the other site a few months back saying that Interstate/Optima is enforcing a minimum advertised pricing policy. In other words to be a dealer you have to agree not to advertise it below the price they set. That's why prices vary so little.
As for made in Mexico, they take all of the difference in cost, none goes to the customer or retailer. It's called capitalism.
I agree... and I make a huge effort to buy American as well. This said, I've had no issue with an Optima that I bought about 4 yrs ago (MIM).