Welcome epson!Regarding your question:
Airfares are cheaper than sleeper fares between major cities because of high volume traffic. But the cheapest air fares between smaller communities, or a big city and a small town, are usually higher than sleeper fares, especially for two or more people.
Busses are cheap, but they are not very comfortable. They don't have sleepers so you can't really compare prices.
Cars are relatively cheap to drive, but when you factor in lodgings and food, it comes out a lot more expensive than a sleeper for an overnight trip. Plus, a train is faster than driving because the train doesn't stop while you sleep.
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-Mr. Toy
The Del Monte Club Car