How does a restaurant value the stock for Zakat purposes?
Ingredients and all foodstuff are Zakatable and are valued at what they are currently worth. If the current value cannot be ascertained, the cost price may be used.
A practical way to ensure the accuracy of stock valuation is maintained is to perform a stock-take on the anniversary date of (or a few days before) the zakat. If there are material fluctuations between the zakat date and on the actual anniversary, the discrepancies should be taken into consideration and then zakat paid.
And Allah knows best!