Generally speaking, it's easy to keep track of the amount you take in when you sell mutual fund shares.
By January 31, your mutual fund or broker should send you a Form 1099-B, Proceeds from Broker and Barter Exchange Transactions, or a substitute, showing you the sales price of shares that you disposed of during the preceding year.
The 1099-B will indicate whether the amount reported is the gross amount or the net amount (gross amounts minus commissions).
If it shows the gross sales price, you have to report that amount (in other words, don't subtract the sales commissions when reporting your sales price in the Capital Gains/Losses section of the Interview or the Capital Gains and Losses Worksheet if you're using the Forms method). Instead, you increase your cost or other basis by any expense of the sale, including sales commissions.
If the 1099-B or substitute statement shows the net amount instead of the gross amount, enter the net amount as your sales price in the Capital Gains/Losses section of the Interview (or the Capital Gains and Losses Worksheet if you're using the Forms method), and do not increase your basis by the sales commission. Because the expense reduces the amount you report, if would be double-dipping to include the same amount in your cost.