if you can't remember what you purchased the house for, call the county records office. They may have the record of the sales price.....or if there was a transfer tax, they can back into what the sales price would have been based on the tax paid.
While it's not best to guess, on the other hand there is the issue of materiality. If you are married and you sold the home for under $500,000 and you qualify for the $500,000 exclusion, just write down $0 as the cost basis- it doesn't matter - there is no tax to be paid, regardless of the true purchase price.
