To actually have a cutoff would mean you would have to pass the curent thru some type circuit that could be over ridden when the voltage got low. Cost would most likely equal the price of the battery pack. Charge fairly often or get a voltage monitor.
A 2,000 pack should last for weeks if:
1. the engine starts quickly
2. the starter box motor, belts and wheel is in good shape
3. everything is lined up so the starter wheel is ONLY touchng the flywheel.
Most of the people that call their starter box junk can't get items 1 and/or 3 right.