If you're able to see the List of Purchase Orders but you are unable to see the Value or Order Value column. This is caused by a permission set against your user settings. To amend this:
From the Toolbar, click System Admin.
Click User Setup.
Scroll to the user who is unable to see the Value column.
Select Company at the top.
Then select the Properties tab.
Then the POP tab.
And finally the Main2 subtab.
Change the setting Allow View Supplier Prices.
With this setting enabled, the user will be able to see the Order value column for Purchase Orders once they log out and back in again.