The sum of $15K + $3K = $18K and that's the basis of the GROSS number of shares in the grant. IF the sale you are working on ISN'T for ALL the shares in the grant THEN you must think in terms of "per share" basis and use the per share basis number times of shares sold as the correct basis to use. Did you sell the same number of shares as the GROSS amount of shares in the grant?
"The way I see it in my 1099 entry is for -$3k after I adjusted the cost basis to account for the income. In my mind, that makes sense since I'm taxed on $15k, but took a "loss" for what I paid for the income."
That doesn't make sense, and I don't even know how many shares you sold. You have a gain in your per share selling price is higher than the per share basis and you have a loss if your per share selling price is lower than your cost basis per share.
