Ugh. This will be time consuming.
Run this query: SELECT ID, TransactionNumber, ItemID FROM TransactionEntry WHERE Cost = 0
In another query window: SELECT ID, ItemLookupCode, Description FROM Item
Export the results ob both queries to separate .csv files.
Get the data into Excel. You probably want to open the .csv with Notepad, use select all/copy then paste into Excel. Use the Text to columns... feature and format any columns that may have leading zeros at TEXT or you will lose the leading zeros. If you lose the leading zeros, things will not go as planned...
Use the VLOOKUP feature to match the ItemID from the TransactionEntry data with the ItemLookupCode/Description information from the Item table data.
Once you have some descriptive data associated with the ItemIDs from the TransactionEntry results, you can start inputing costs into a new column. Once you have all of the cost info entered, you can build a text formula that looks like this: ="UPDATE TransactionEntry SET Cost = ",R1C1," WHERE ID = ",R1C1
The result of this Excel formula will be: UPDATE TransactionEntry SET Cost = XX.XX WHERE ID = Y
Use the Fill Down feature to create a separate query for each ID from TransactionEntry.
WAIT UNTIL THE STORE IS CLOSED. BACKUP YOUR DATABASE.
Copy the array of queries into a Store Ops Admin query window. Run.
Test the results with a report.
If you're happy, great. If not, restore from backup and try again.
I hope that helps...
Tom