There is no criteria available to come up with a search to show only those items with blank base price. The criteria Pricing : Rate (Pricing fields > Rate) in an item search is checking all price levels and not just the base price.
As an alternate solution:
Navigate to Lists > Accounting > Items
View = Sales
Type = Inventory
Click Export - Microsoft Excel
Open the report in Microsoft Excel and filter the list to show those items with blank/null base price.