If this is your primary residence your gain may be excluded from income anyway, but the best thing to do, is try to reconstruct the best you can.
Perhaps check with the county if they have records back from 2004, when purchased. Just do the best you can and document what you did to arrive at a cost.
Then over the years you owned it think about what you may have done, such as adding a deck, a room addition, and other items that can be added to the basis.
If you make no attempt to reconstruct this you will have to use $0 as cost. However, IRS can be very good about accepting a reasonable basis if you can show what you did to arrive at a cost (only show if asked however as this does not go in with the tax return).
