If you simply sold the original 100 shares then you would report the cost as 100*55 ($5500) plus commissions that you paid on the purchase. However, if for instance there had been a split (say 2:1), and you now held 200 shares and sold 100, the cost of the 100 that you sold would be 1/2 the cost of the original 100 that you purchased (100*27.5 plus 1/2 the original commission). Had there been a spin off and you received other stock or cash as a result then the "cost" calculation can get more complicated and depend on the particular rules of the reorganization.