I've got the Everstart Maxx 29 series from wal-mart for $76, if you've got another battery you get $9 taken off that - I just bought it about 2 weeks ago only because I like the idea that I can exchange it anytime, no questions asked within the first 18 months - only used it for about 15 min though...
I have used the Duralast Marine battery and I use that on my 12' with a 34# motor guide... it lasts all day long, but if I stay running wide open and keep moving spots... it will die down in about 5 hours... then again, it is 6 years old this year.