We did this last summer. The 35AH battery we're using cost $64 at the time. I probably ended up spending more per cycle, but I'm guessing my out-of-pocket at the time was lower (and it was kind of a gamble as to how much we'd actually use the rig).
The other thing to consider is that lead batteries don't mind being floated at max capacity for long periods of time. Lithium batteries like to stay closer to 90%, unfortunately most chargers just blindly charge to 100%.
