[header] How to adjust cost basis after a merger [title] Determine the original cost basis of your investment, including any commissions paid. [step] In the example above, you paid $5098 for 200 shares of company a at $25.49 per share. Suppose you paid $10 commission for this transaction. 
Your original cost basis is therefore $5108. Dividing by 200 shares of company a, your cost basis is $25.54 per share.