I have filed my return and paid the amount due, I I reported an incorrect cost basis in the IIRS's favor . Can I file an amended return and what does it cost?

[KittyM]

If you filed using TurboTax, amending is free.

Before you can correct (amend your return), you will need to wait until your return has been accepted and processed by the IRS.  It generally takes the IRS 10-21 days to process a return, but it can take longer in certain circumstances. TurboTax will prompt you to amend your state return when you redo your Federal return.  If the error affects both returns, you will need to amend your state return as well.  

Here's how to Amend My Return in TurboTax.