Intended User: People with a backyard and don't plan on moving the thing around much.  
Advantage:  Cheap
Disadvantage:  Heavy (240 lbs), Post possibly not repairable over time.

Go to the hardware store and get a "4 by 4" fence post or yard timber.  It may be 8' long, just have them cut it to 6' long.  Put the plastic bags along the bottom to contain the concrete from going out the bottom of the tire.  Put the post vertical in the tire being held steady by some scrap wood or rope.  Next mix the concrete and carefully pour into the tire.

Parts List: 
Fence Post or Yard Timber from Hardware Store $2.50

4 Bags of 60lb Quick Crete concrete mix. $2.00 per bag.

Old Tire free from tire repair store.

Something to mix concrete with, plastic bags, duct tape.

Allow the concrete to dry for however long.