How much money are you talking about to begin with? My wife CPA ran into a similar problem and did a long spreadsheet computation to figure out the cost basis. Like you, she didn't have the info so she worked from the monthly statements she had. Honestly, you may not like my answer but if the amount of money is not a lot, I'd say just do the best you can do and let it go. I mean if they want to audit you, let the IRS tell you the cost basis then. Sorry if my answer isn't the academic answer you might expect. I'm not saying to make up a number but rather give it your best computation and go with it. If you get audited, at least you can show them what you did and let them tell you the correct number.
