What's the meaning of "Lose money" on this sign?

This may be a case of shoddy translation.

Inputting the simplified Chinese characters into LineDict returns 亏本卖, which apparently means to sell something at a loss, i.e. the merchant will not earn back the cost of the product from the money earned on the sale.

According to Google Translate, jualan rugi is Malay for sales loss.

Thus, the intended meaning seems to be that the price is so low that the shopkeeper is losing money, and therefore the consumer is getting a very good deal indeed. But lose money is not the way to express this in English, and as you observe, will be taken quite the opposite way—that the customer will lose money.

The English equivalent would be something like below cost, i.e. the merchant's cost. I'm not familiar with any one conventional phrasing, as it isn't a common thing to advertise in the U.S. or Canada. Here, that is mainly a selling point on more expensive items like cars or major household appliances, which might be billed as below factory invoice or below dealer invoice or some such. Selling below cost on a regular basis to drive out competition is actually against the law.