Dou was a type of unit of measurement. One dou was equivalent to six catties. Half a dou was equivalent to three catties, which was around three taels of silver.
One tael of silver for one catty of salt was indeed expensive. It was equivalent to many commoners' one month's salary.
He remembered a nutritionist on Earth once said that a person should have a daily intake of 5 grams of salt. That would be equivalent to 3 taels of salt every month. A family of three would need around one catty of salt every month. This was very affordable for Earthlings but extremely expensive for Dayao people.
Even the cook at the beast park used salt sparingly while cooking. That was because meat was the main source of food in Dayao. Firstly, the price of salt was very high. Secondly, they could not eat too much salt.