You can achieve above result by following the below steps:
Step 1: Create custom lookup field on Inventory Assignment object
Step 2: Create automation to populate Transaction Item lookup field
Get the first set of Id from a field name Inventory Assignment Id & populate into Transaction Item lookup field.
You can create this automation either using workflow / Process Builder / Trigger.
Step 3: Create Report Type
Edit report type layout to display more fields via lookup
Select Transaction Item.Unit Price & Inventory Location.Average Unit Cost field. Save the layout.
You are now ready to create your report.