A single member LLC is not a taxable entity - all income would be passed to you and will be reported and taxed as self-employment income.
Here is a raw estimation of your tax liability assuming - you are single, standard deduction, do not have any dependents, no other deductions or credits, no other income, $400,000 is your NET self-employment income (after all deductions)
-- self-employment tax - $23,956
-- income tax - $110,645
Total estimated tax liability - $134,601
What might reduce your tax liability?
-- if you have any deductible business expenses - your NET business income will be less - and both your self-employment and income tax liability with be less;
-- if you have a retirement plan - you may make a contribution into that plan and that will reduce your income tax;
-- most likely you will itemize - and that will reduce your income tax liability.
Let me know if you need any help.
Edited by LEV on 7/5/2010 at 2:31 AM EST