What percentage of that goes to payroll, SSN, employer and employee taxes, etc...

Any idea how ---from remaining 80%..
     employer will generally take payroll charges --10-13% ( depends on company size)
   Then goes your income tax like SSN,Medicare, State etc..--20-25-30% ( depends on which state you are in ).

So ultimately you get around 50% of the billing rate in your hand.

Guess Who am i ?

Don't forget about insurance, vacation time...