they are so NOT costly (and usually during on sale nov-dec) that i still just buy new ones each year. this year i'm paying about 50 cents for the Baking Soda and about $1 for the BP. since i use more BS, i recyle it 3x/year. when i renew my baking soda stash, i toss the fridge container down the disposal drain and let it soak, and refill my everyday baking soda into an 8-oz Ball jar, and then also put the open box back into the fridge.
in my view, if i'm spending some substantial $$ on the other ingredients, then it would be preposterous not to buy a fresh batch of each.
What one understands is only half true. What one does not understand is the full truth. ~ Zen saying