I work part-time for an auto parts company and can get a pretty good discount on batteries. The company sells a 6V wet cell GC2 with these specs:
5 Amp Hour Rate (Ah): 174Ah
6 Amp Hour Rate (Ah): 178Ah
20 Amp Hour Rate (Ah): 230Ah
Reserve Capacity At 25 Amps (min): 448 Minute
Reserve Capacity At 75 Amps (min): 120 Minute
Electrolyte Composition: Acid
Terminal Type: Top Mount
Voltage (V): 6 Volt
Battery Type: Deep Cycle
Polarity: Right Positive
This battery I can buy for about $80 each + tax.
They also have this one an AGM 6 volt:
Group Size (BCI): GC2
Cold Cranking Amps (CCA): 680 CCA
Voltage (V): 6 Volt
Terminal Type: Top Post
Reserve Capacity (AH): 380Ah
1 Amp Hour Rate (Ah): 103Ah
100 Amp Hour Rate (Ah): 220Ah
20 Amp Hour Rate (Ah): 190Ah
3 Amp Hour Rate (Ah): 145Ah
5 Amp Hour Rate (Ah): 163Ah
6 Amp Hour Rate (Ah): 167.8Ah
8 Amp Hour Rate (Ah): 173.7Ah
These batteries are specially designed for the long, deep discharges typical of marine, wheelchair, RVs, emergency lighting, golf cars, other deep cycle applications as well as engine starting. The absorbed glass mat (AGM) design provides maintenance-free service and ensures that they are spillproof and leakproof.
This AGM I can buy for about $170 each + tax. I like the AGM batteries over the wet cell type, but not totally sure I want to spend the money.