How to calculate cost basis for RSUs that are vested and never sold?

The 1099-B provided by E*Trade is for the sell occurred on vest date i.e, sale proceeds from the sell of 20 shares in market. The net 100 shares you received after tax should not be on the 1099-B. The cost basis of those 20 Shares is the award price on which you received RSUs. In your case you received it as an award of 0 price.

The rest 100 shares will not be reported until you sell them. Also, the cost price of the 100 shares will be the fair market value on the vest (this is usual the close price of stock from one day before the vest date or sometimes the execution price of the sold 20 shares).

Please note, as these shares are result of RSUs awarded by ABC Inc, you can post the question to the Head of Compensation and Benefits team of ABC Inc. You can also request the team to confirm if the income from sell of 20 shares has been reported on W2 form.

I've used E*Trade for RSUs for many years, and they have never reported a "cost basis". I've always had to look up the current market price at the time of vesting as my cost basis. You definitely should not use $0 since shares were sold to account for the "income" you received via these shares. If you use a $0 cost basis then you'll be double-taxed.