Your basis is the amount of your nondeductible contributions remaining in your traditional IRAs, so your basis would be $10,000 assuming that you have made no distributions from your IRAs in or after the year for which you first made a nondeductible traditional IRA contribution. Investment earnings do not add to your basis.
Some basis is distributed with each regular distribution from any of your traditional IRA accounts, but some basis will always remain as long as the total of the year-end balances in all of your traditional IRAs is not zero.
