Another possible cheap source for grit is the ground. If they have access to the ground they are probably finding their own grit. That’s part of why they are always scratching and pecking the ground, they are looking for grit. It’s possible they won’t get enough from your ground but most do.
Another good source is coarse sand or small gravel, about the size of a green pea or smaller. If you have legal access to a sand or gravel bar in a stream or river you have a good source. Just throw it on the ground in the run. They will enjoy scratching for it.
The grit you buy is granite. It is a by-product of a granite quarry. They just sift the rubble through screens to get the right size and bag it. Granite is very hard and will last a long time. A pea sized chunk may last as long as a month.
They can use other rocks besides granite for grit. How long it lasts depend on how hard it is. All grit eventually gets ground down to a fine sand and passes on through their system and out the back end. They need a continual supply but just how much depends on how big the chunks are and how long they last.
You can use gravel from a driveway but there is a caution with that. If you live where the roads are salted in the winter that might contain a high amount of salt. Chickens can’t handle a lot of salt. Also a lot of driveway gravel is limestone. That is a source of calcium. If you use that for grit I’d suggest you not feed Layer but instead feed a Grower, Starter, or Flock Raiser low in calcium and offer oyster shell on the side to keep from overloading them with calcium. I’m probably being overcautious here. Chickens raised where limestone is the natural rock use it for grit all the time and from what I’ve observed don’t have any problems from that. But those chickens in my experience were free range chickens not fed any chicken feed containing any excess calcium. Their egg shells were still quite hard.