I know it is a bit late, but Aldi's annual camping equipment sale includes 12V 40Ah AGM batteries for $100. They are about the right size for smaller 7 1/4" or larger 5" gauge models. This price is far less than from the usual sources.
It is funny how the Aldi idea of "camping" includes so much heavy equipment. If you took up all their suggestions your 4WD would have no room left for passengers. I don't have a 4WD but I do have trains to carry my batteries around.
They have them again this week, but unfortunately they are now rated at 38Ah rather than 40. So that's a 5% reduction in capacity with no reduction in price. Meanwhile lithium batteries are getting cheaper year by year although still about twice the cost per kWh as lead batteries,