As far as cost if you decide on a lead acid battery [and weight is not an issue] Wal-mart has several deep cycle. The higher the amp hour rating the longer the charge will last.
Remember to always charge your battery as soon as you get home and check the water every month or so. Then you will need a charger that is self regulating so the battery does not get overcharged and boils your water away. On second thought do what Mike said.
There was a great article in the In-Fisherman magazine about all the pro's and con's on all the different batteries.I don't remember what issue,but maybe you can find it on there web site.