You got it wrong. Tax got nothing to do with Inventory item value. The tax amount goes to Tax accounts. So on inventory tab you will only find the value of item without tax.
Still if you want to see value before and after tax then you need to find it on other tabs like Payments and Purchase Invoices.
However, if you click on Total cost figure of an item you will get the Unit cost value of an item which is again before tax.
