50 cents each, but you can't buy just one
They sell them in sets of two for a dollar. It is called a "Square ratchet"
About not breaking the next one:
The position of the ram is pretty much irrelevant, though it is better if the indexing arm is not on the twisted part of the indexing rod.
The inside of the indexing arm has notches inside, on the upper surface only. Those notches engage the square ratchet only as the ram is moving downward. When the ratchet is held still and travels over the twisted part of the indexing rod, the rod is forced to turn. There is supposed to be a little bit of drag between the indexing rod and the ratchet to ensure the ratchet engages the notches.
There are two ways to break the square ratchet:
If the turret head is held still while the ram moves over the twisted part and the ratchet is engaged with the notches (moving downward), the ratchet will break, guaranteed. This saves the indexing arm from breaking; it costs $6.00.
The more common way to break the ratchet is to turn the turret (and the indexing rod to which it is connected) while the ratchet is engaged with the notches inside the indexing arm. ((Here's where a lot of people miss the point.)) The ratchet is engaged if the most recent movement of the ram was down. It does not matter what position the ram is in. The direction of most recent movement matters.
One other caution: If the ratchet is on the twisted part of the indexing rod, turning the rod/turret could conceivably, through camming action, move the ratchet up and into engagement with the notches. So this might be a third way to break it.
Another advice: My spare square ratchet is stored, slid onto the indexing rod, at the very top. It is out of the way, out of sight and unlikely to be lost when/if I need it. There is probably room for several there if one were inclined to have that many spares.
Last advice: The ratchet is not reversible. It is possible to put it on upside down. Be observant when you take the old one off. Describing which is the top and which is the bottom is not easy and seeing the notches in the indexing arm is not easy, either (black plastic makes it hard). It will work upside down, I am told, but not for long. Engagement with the notches is tenuous and the corners get rounded off. Bad news.
Good luck.
Lost Sheep