The size or capacity (not physical size) of a gas furnace will have a dramatic effect on the cost. In residential furnaces the sizes include a range from 40,000 BTU up to 140,000 BTU. The larger the furnace, the most the furnace will cost.
To size which furnace is required to heat your home many factors come into play. The geographical location of your home, square footage of your space, the height of the ceilings, insulation of the home and even the type of windows that your home has.
The last factor to consider is the size of the ductwork inside your home. Having too small of ductwork in your home for the size of furnace can greatly effect the performance of your new furnace significantly, even worse it can lead to costly repairs over time.