I guess it depends on how deep your pockets are and how long you want the battery or batteries to last.
I got lucky and got four 6 volt forklift style batteries for almost nothing.
Sure, they wont do 8 hours work in a forklift anymore, ( thats why they got rid of them ) but I dont have a forklift to put them in anyway. They weigh about 350 pounds each but I can expect about twenty years life out of them so it was a good way to go.
For a smaller system, look at plain 6 volt golf cart batteries.
Not alot of money to buy them with and they will last along time if you dont over discharge them.
Stay away from any battery that lists 'cca' ( cold cranking amps ). They arent true deep cycle.
Luck, Dewayne