We have a Teagle and that is a good question.
It is 2 years old and we bought but did not get the knives installed this year. We put through about a bale and a half a day once we start feeding of ridiculously tight bales weighing about 2300 lbs. I am running the baler this year so they will not be as tight. We also used it to bed the barns. That is the advantage of the Teagle.
Our costs have been fuel, an alternator, and 2 air filters for the tractor because the dust is hard on stuff. It does not use any more fuel than if we were cutting hay and it runs for a much shorter amount of time. We have repaired a couple of tires because of where we were driving.
I do think you need to be aware of how you are putting up hay. If you were cutting extra close or had tons of rocks and hard stuff you will want to change things a bit.
Our savings on hay far outweighs the cost of running it.