How much does it cost to move a furnace from the attic down to the first floor?
Six Things to Consider:


Electrical - Generally easy to move
Natural Gas (if it is a natural gas furnace)
Output Air - might require one big duct between old and new but there may be alternatives.
Return Air - similar to Output Air
Combustion Exhaust
Connection to air conditioner (if you have one) - typically a couple of pipes for input and output.


Except for electrical, all of these require some real planning to make sure they can be done safely and effectively and at a reasonable price. But hard to guess as variables include:


type of furnace
access to old and new locations 
walls between old and new locations