Cost/Unit, Supplier, and Date Purchased can be updated individually on the Sub MSKU pages.
First, click on List at the top of your screen and select either FBA or Merchant, depending on what channel your item is in
This will take you to the Accounting view of the Main Inventory page
You can search for the item you'd like to update by using keywords, Title, ASIN, MSKU, or scanning the UPC in search box showing in the upper right hand corner.
Click directly on either the Active Cost/Unit, Supplier, or Date Purchased.
It will take you to the editing mode of the active sort sequence in your Sub MSKU page. Fill in your accounting information and click 'Save'.
If you have a lot of items to update, you can also import your buy cost in bulk using a .csv by following these instructions to save time!