I believe the Sam's Club 6V Golf Cart batteries are the best deal going right now.
However, things change quick.
From what I can last recall (maybe off a bit).
Two 6V "T-105s" rated at ~200AH from Sam's are about $145 if the buyer brings 2 batteries to trade in (and the local Sam's takes a 7AH SLA the same as an 8D, "A battery is a battery" he said. Bringing 2 scrap 7AH SLAs saved me $20).
A 110~115AH marine battery is ~$95, with a trade in. Short lived and low capacity.
A pair of L-16HCs is ~$750. ~50% more capacity than 'T-105s' but only last 2~3 more years.
The math tells me it is a LOT cheaper to use Sam's golf cart batteries. Dollars per year, amp hours of capacity, etc.
Sam's has 0% problem exchanging a bad (car) battery. My experience.
Trojan 101% REFUSES (many people's experience) unless the operator can prove it was never abused even once. Try to prove that 100%!
It depends on the charge and discharge amps too.
The stuff I deal with is 3 to 25A max charge, 100ma to 50A discharge.
L-16s may be better with higher rates. I don't know.
End rant.
Brian: Buy the batteries.
G-