What is the average number of homes with at least two staircases where
one is a main entrance staircase? Stairs require approximately 100
square feet of space in a home, while a residential elevator can take
up as little as 24 square feet. I have been informed by builders that
an average high-end staircase (rod iron/wood combination) can run as
much as $15,000.

I'm not sure what you mean by main entrance stair case except in
apartments where the unit is entirely on the second floor other than
the stair case. In single family homes, the stair case is typically
near the front entrance, so that could qualify as a main entrance
stair case, even if this is a split/tri-level house with several 1/2
or 1/3 length stair cases. I presume 4 or 5 steps from the ground to
the first floor level, does not count as a stair case.
If the 100 square feet, is shared by a set of steps to the attic
and/or the basement should we concider that 2 or 3 stair cases for
purposes of answering your question?
 If the elevators can be installed for $15,000, and maintence is low,
long term, low noise and less than one minute per average use, I
believe as many as one percent of the new construction homes,
apartments, condos and town houses would use them as a suppliment to
narrow low cost stair cases which are required in most locals in case
a fire etc disables the elevator. Are you thinking the 48 USA States,
world wide, or some other group of countries?   Neil