I just went to a local junkyard and bought 3 8D style batteries, like these, for $60 a piece:
Mine are a tiny bit different (two "Federal 908D" and one "Champion 8D"), but are about as wide as 3 car batteries, and weigh 130 lbs each, etc. I was curious whether these are suitable for solar energy storage, and my approximation is that if so, it would be around 750-800 amp-hours (I've read 8D batteries are around 225 amp-hours each).
My worry is that they are some hybrid deep + starting type of battery that won't rate well for deep cycles, since they refer to 'Cold Cranking Amps' (around 1500 from what I can tell) on the top. I know these aren't the steady amps. If I can at least get some life out of them, ie 1 year, etc then that'll be fine.