Lowest cost is a pair of six volt Wet cells (GC-2) From San's club
Least worry but most expensive is AGM
In between is a maintenance free "Trolling battery" if you can find one. Wal-mart advertises but I've never laid eyes on. This should not be a Marine/Deep cycle, but a true deep cycle.
Note also that with proper maintenance, Life expectancy (Total battery life not time between recharges) is the opposite of price, Lower lasts longer in my experience. (Assumes all are of equal quality for their class)
I put AGM's in on their own loop back in 2006, Replaced them 2012, The original (2005) Wet cells are still going strong.