There isn't a report within Procure Wizard that shows items that are available on a supplier's price list that you haven't purchased.
There is no report to show if the end user purposely chose a more expensive supplier.
Providing you have access to reports, there are reports relating to product purchase history, please refer to the below reports.
Goods Purchased Report
Goods Purchased Analysis
Goods Quantity Purchased
Orders Placed by Purchaser
Spend Analysis located under Managed Suppliers