Let's say I buy 20 shares of a stock at $20 a share, and I pay a $4.50 commision through Zecco. My actual cost for this group of shares is $404.50 for tax purpose. Now, let's say I decide to sell 10 of those shares for $25 a share the following month, and the commission is free. Where do I allocate the $4.50 purchase commission for tax purpose? I made $250 revenue on my sale. Do I subtract the entire commision right away, where $250 minues $204.50 would equal a profit of $45.50? Do I prorate the commission proportionately to the percentage of the initial stock that I ended up selling, where $250 minues $202.25 would equal a profit of $47.75? Or do I put off the entire commission, where $250 minus $200 equals a profit of $50?