in the winter, it's really a question of how much heat you're getting for the money... in the summer and above comfortable temperatures, anything that draws power gets evil glares from me.
Watts to Btus (IT)/Hour Conversion Calculator
that should give you a rough idea of how much heat you're generating, you'll have to figure out cost on your own since power rates are so incredibly variable depending on where you're located, time of year, even time of day for some people.
in my personal digging:
1500 watt electric heater = 5118 BTU/hr assuming it's running at 100% DC. BTW: electric heat is so close to 100% efficient, it's hilarious when ads run for those infrared heaters that claim to be better than a regular electric heater.
http://www.handymath.com/cgi-bin/electric.cgi
it costs me 11.6993 cents per KW/H last month for power.
if it ran at all times(24 hours X 30 days = 720 hours), it would end up costing 126.35 for 3,684,960 BTUs.
compare that to how much it costs you to run the gas furnace and you'll see if you're actually saving or losing money.