Borax exists in 3 forms: a pentahydrate, hydrous & decahydrate. Borax is a widely used chemical with such diverse uses as detergents & ant poison. If you were trying to optimize the maximum amount of borate ion B4O7-2 for the cost, which one would you buy?
Borax pentahydrate (Na2B4O7 •5H20) $5.29
Borax decahydrate (Na2B4O7 •10H20) $3.98
Anhydrous borax (Na2B4O7) $7.49