Your SIMPLE IRA is a type of traditional IRA. IRAs do not have "cost" basis, but traditional IRAs can have basis in nondeductible traditional IRA contributions in the aggregate of all of your traditional IRA accounts.
If the SIMPLE IRA is your only IRA account and you never rolled any money from another type of traditional IRA or another type of employer plan where you could have made nondeductible or after-tax contributions, your basis in nondeductible traditional IRA contributions is $0 because neither your elective deferrals nor employer contributions to a SIMPLE IRA are nondeductible contributions.
