cost per use breakdown of my skincare - changed how i buy

built a spreadsheet of products i own by price / grams / expected daily use / shelf life. most 'luxury' serums are $3-6 per use. most drugstore actives are $0.10-0.30. when i compared result-per-dollar the drugstore stuff won every time except for 1 vit C that was worth it for the stability. not anti-luxury, just pro-math.