i'm not familiar with the pricing for the concrete countertop
products...but generally all concrete and "cememt" products come in
40#, 60#, 90# bags or larger...but they are really cheap! just buy
the bag that is "big" to you, and keep it in a really dry place...and
you will have a lifetime supply!
you may e able to go down to your local concrete-mixing facility and
talk to the guys..htey may just give you a few pounds of portland
cement if you tell them what you are wanting.