I'm using Purina brand Oyster Shells in 5# bags, $5.99 at 
TSC.
Chickens eat as needed.
I recently bought 
Manna Pro brand, 25# for $9.99 at 
TSC. I haven't offered it to the hens yet, but I opened the bag and it looks Ok, same size as the Purina brand. But it's a mixture of Oyster Shells and Coral Calcium. Purina is just Oyster Shells. Calcium is slightly lower with the 
Manna Pro, 33/38%, Purina, 36/41%.
Granite Grit is cheap if you get the 
Manna Pro brand in the 25# bag $8.99 at 
TSC. DuMOR brand 5# $6.99.
Get the Chick Grit 5# bag for Chicks. Offer it from 2 to 7 weeks, then mix in the larger size Poultry Grit after 7 weeks.
		
		
	
	
Granite Grit lasts a long time and is good insurance that your chickens can easily find the proper size and hardness of stones year round.
I still have over half of a 25# bag after 2 years with 5 hens most of that time. So figure about a Pound of Granite Grit per chicken per year.
Oyster Shells consumption will vary widely depending on what you feed. Layers feed only, no scratch grains, corn or kitchen, table scraps and no free ranging. They may not consume any Oyster Shells. 
If you don't feed any layers feed and instead feed an All-Flock, Flock Raiser or a Non-Medicated Starter Grower, or you give them lots of scraps and treats or they free range all day they will consume more.
A 5# bag lasts me about 11 months for 5 hens fed a layers feed, with scratch grains/mixed bird seed scattered on the ground twice a day, and an hour or two of free range time a day weather permitting. GC