Around $100 for a new 27" rear flip/flop wheel.
The cog and lockring will set you back another $30 or so.

You don't need new cranks or a new front chainring. All you have to do is pick the ring that you are going to use, and use single chainring bolts ($5) to use that ring.

Scroll down to 27" fixed gear rear wheels:
http://sheldonbrown.com/harris/wheels1.html