Calculate cost basis after stock conversion

Ok ... back to third grade math class. 

You owned 200 shares @ 10 share = 2000 cost basis

Now you own 150 shares  with a 2000 cost basis(what it was worth at the time of conversion is immaterial to your basis)   ...  2000 divided by 150 = 13.34 per share

Sell  15 shares x 13.34 per share = $200 cost basis

The cost basis is what you originally paid, or $2,000 in this case (200 shares x $10.) I think you mean you sold the entire investment, if so your cost basis is $2,000.