When you use cruise control you actually will save money as long as you're not speeding with the cruise control on.
The cruise control is good for saving gasoline as your vehicles engine will remain at a constant throttle position and speed.
So instead of moving the gas pedal to speed up and slow down which uses more gas the cruise control will save gasoline and you can relax your foot and leg as you cruise down the highway.
Just always be aware of your surroundings and be ready to step on the brake pedal when needed.
Cruise control is a nice feature and I love to use it myself when driving long distances but you still need to be alert as you can't just put on cruise control and forget about it.