68° F is a really high thermostat setting. Setting it to 65° F would probably result in a $100 drop, minimum. If you could survive 60° F overnight and 64° F during the day you'd probably get into the $700 range.

Something sounds pretty off with your system or insulation though, since a modern energy efficient setup with proper insulation and windows/doors shouldn't cost more than $500 to hear a unit of that size to that high a temp. I'd probably look into Mass Save programs to bump up your insulation, fix drafts, maybe purchase a heat pump, etc. Assuming you own the condo that is.