I'd skip the calcium carbonate(cuttlebone) as it would raise KH as well as GH. Magnesium sulphate (MgSO4) or Epsom salt will raise GH without altering KH. Used in a ratio of 1:3 with CaSO4 (Calcium sulphate) or plaster of Paris. Use 1 part MgSO4 to 3 parts CaSO4 to make your own GH booster.
Expense isn't so much of an issue as effort and time because the cost of these materials would likely last longer than a $18 bottle of shirakura CA+.