Don't think too much about this. For example, I remember that the wholesale price of selling popsicles at 50 cents is 3.5 cents, and the wholesale price of selling popsicles at 1 yuan is 7.5 cents. You buy two 50 cents, he earns 30 cents, one yuan, and he earns 2.5 cents. In other words, if you have 1 yuan, buying one 1 yuan will directly earn less than buying two families with 50 cents. This is because of the cost. Although the profit is low at low price, it sells more. Under the same quantity, the more you buy, the less he earns personally. However, the total profit has increased.

If you want to better explain the phenomenon of washing powder, I think you should understand the local consumption and purchase level, the relationship between price supply and demand, the relationship between cost and profit and so on. In order to comprehensively consider the pricing problems of 500G and1000g, if you have relevant data or want to explain this phenomenon from the perspective of mathematics (economics), I don't know to ask after the study.