If I had to liquidate my 401k and IRA today (total balance =$100,000), how would I calculate that and approximately how much would I be left with after taxes and penmalties?

Assuming you are under 55 years of age, you would pay the following:

1. Ordinary income tax on your pre-tax contributions and all growth of the accounts. For most people this means tax on the full withdrawal. Not knowing your marriage status or how much other income you have, this $100K withdrawal would most likely put you in the 28% or 33% tax bracket.
2. State or local income tax, the amount of which depends on where you live.
3. 10% early withdrawal penalty.

So you would be left with around $50K - $60K to spend. If you actually did this the plan admistrators/custodians would automatically withhold 20% for federal income tax, but they do not automatically withhold state/local tax or the early withdrawal penalty, so be sure to save an adequate amount so you can pay your taxes next April.

One other point - as your Adjusted Gross Income gets to $150K or higher the advantages of many tax deductions and credits begin to phase out, which means the effective marginal tax rate may actually be higher.