Your total cost basis information will be made available by February 15 on the Form 1099-B.
You'll be able to access it directly from "Documents". The 1099-B Summary page will list your total cost basis information, broken out between short-term and long-term transactions.
Detailed cost basis for each sale transaction will be available on the same Form 1099-B after the initial Summary page of the form.
Note that you can see an estimated tax impact preview within your Betterment account when attempting to initiate a withdrawal from a taxable goal, prior to finalizing your withdrawal request.