Inventory > Import Product Info > Import Vendor SKU/Prices
Vendor SKUs and Vendor Prices can be added to a particular product in bulk using the the VendorSKU.txt template. Purchase orders created with that vendor selected will auto populate the Unit Cost field with the vendor price.
The file can be created in excel and saved as a .txt file.
The template columns are as follows:
Note: there is also a way to update these values from the general BulkUpdateProducts function, but it is more limited since that works with just one row per product while this approach can have multiple vendor prices for the same product on different rows of the same file.