Space heaters v/s whole house

My house is about 2000 square feet and the downstairs gets cold. There are 2
bedrooms and a large empty room there, the kitchen, great room and 2
bedrooms are upstairs. Itsusually warmer. The thermostat is near the kitchen
so that throws things off, the heat wont come on till its near freexing
downstairs.
ANYWAY
There are 4 bedrooms, Ispose the cheapest thing would be to make the boys
share a room downstairs and use one heater and make my daughter share mine.
But that isnt going to happen
AM I saving money by using a few small space heaters and not heating the
entire house? Or do 3 small heaters use as much energy?
I dont want to heat downstairs because to get it warm would require heating
it to about 80 up here.
Help