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Wow! Trojan batteries are not cheap. I think Les Schwab might be in my price range.
One of the reasons I went with the 12's versus 6 is that I was concerned about someone else wanting them. As stated the 12's are much cheaper, the solar panel I have was $100 and I didn't mount it permanently for snow/freezing rain.
FYI there are only like 3 battery manufactures... Different brand are marketing... Costco normally has a good deal on batteries... Same kind as everyone else... And the heavier the battery the better.