Obviously keeping one room at 75 versus a house would be cheaper.

I keep my house at 68 during the winter and the snake room at 75, I could not imagine keeping the entire house at 75.

Now there are other options as well that could be cheaper such as installing a heat pad on the cool side and keep your t-stat to where you can achieve 75 and above.

It really depends on the room size how hard a heater would have to work based on the original room temp, how many animals are in the room etc.