You can use each separate commission for each transaction -- in determining the cost basis, and then in computing the sales price.
For example:
The tax basis of stock you purchase is what you pay for it, plus the
commission you pay. Say you buy 100 shares of XYZ Inc. at $40 a share,
and you pay a $100 commission. The total cost is $4,100 and the tax
basis of each of your shares is $41.
If you sell the 100 shares for same $40 each, and pay $100 commission
on the sale, you have a $200 loss—your $4,100 basis minus the $3,900
proceeds of the sale.
