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The cost basis would be whatever you paid for the 3 shares that didn't get converted in the reverse stock split.

If you bought all the shares at one time, then it is just 3 times the stock purchase price.

If you bought shares at different times, then it can get a little complicated. You can choose to use the price of the first 3 shares, the last 3 shares or use cost averaging to determine cost basis.

It is equivalent to selling 3 shares for $11.30 before the reverse split.

If and when you sell the remaining shares, you will need to calculate the remaining cost basis by subtracting the cost you used for the 3 removed shares. 

I hope that helps.