The cost basis for the sales to cover expenses can be determined from this document for GLD and this one for SLV.
If you take the number from the third column and divide it by the second column, that will give you the fraction of the commodity used to cover expenses. Multiply that by your total cost basis to get your cost basis for the sale.
Then for the next month, you reduce your cost basis for the sale from the previous month. @Jack100000 has suggested an excel spreadsheet to calculate this for you in the bottom of this post related to GLD shares.
This can be labor-intensive. For transactions with no basis reported to the IRS, they will never complain about reporting a $0 basis. Entering anything else could lead to further questions, which is why you do want to keep these documents if you choose to do the work.
@msb2
