It isn't expensive at all. It's usually crushed granite. It's insoluble so that when it lodges in the gizzard, it will stay there till it dissolves from stomach acid while it aids the gizzard to grind feedstuffs. For chickens, there are 3 sizes. Chicks get #1 (1/16”) grit till about 3 or 4 weeks. Adolescents get #2 (3/16”) grit and birds from about 15 weeks get #3 (5/16”) grit. #2 is probably big enough for adult bantams.
There's a larger grit for adult turkeys.
Unless you only feed mash or other finely ground grain, it's best to provide some even if they have a large pen. Giving it to chicks helps develop the gizzard and may guard against pasty butt.