really don't want to do the math again but while ago I figured $5-10 to supplement a pail from chemicals fr littlesilvermax.
From another source, recent post
here had bulk Ca at $113 for 50kg ($2.26/kg). I change about 20g a time, supplement Ca using about 1oz by volume of the Dowflake, or 33grams. For a pail (160g / 8 changes), about 0.26kg total.
Use lots more Mg about 180gram per change, 1.4kg per pail with LSM selling MG around $5/kg