Why don't people jump rope instead of using the treadmill, it burns way more calories and I find it easier. It is also much cheaper,and you can do it where ever you want. Very easy cardio.
Jumping rope is a pretty high impact exercise and is not "easier" for a lot of people. Many people who are extremely overweight can't jump rope due to the impact on their joints as well as the simple bouncing motion. Many women (like myself) have a hard time jumping rope because even with the best sports bra, the bouncing/impact to the chest is painful.
What's best/burns more calories/is cheaper is purely subjective. If cheap is the only criteria, then save the $4 for the rope and start walking. Find a neighborhood with hills and walk that. Climb stairs. All of those are cheap and easy as well ... and significantly less impact to the body.
And burning calories is a function of how high your heart rate goes and your current weight far more than what machine or tool you use to do it. I guarantee you there are people who can "outburn" a jump rope on the treadmill simply by ratcheting up their level of exertion. Heck there are people who can outburn a jump rope by running up and down the stairs in their house.