If something is “cheap” it is “not expensive” or “something that doesn’t cost much money”.
“Dirt Cheap” is Super cheap compared with other similar things. It means incredibly cheap, at least relatively speaking.
I could say, “The food in Thailand tastes amazing, is very healthy, and is dirt cheap“.
I could also use it in a way to make a comparison. ”Computers are dirt cheap now compared with 10 years ago”.
A Class Act
A “class act” is a very good and quite a high level slang term to be able to use.
If someone is a “class act” it means they are very “classy”. It’s actually a very positive term if someone is a real class act. They are a great, wonderful, and classy person etc…
BUT… Here is how the phrase is ALMOST ALWAYS used. It’s almost always a sarcastic one. That means, people almost always use this phrase to describe someone who is NOT a class act. They are instead the opposite of a class act.
So imagine someone who is loud, rude, and stupid. Maybe they are overconfident at the same time. That would be a terrible combination of character traits for someone to have, but I’m sure we all know people like this.
This loud, rude, and annoying person could be referred to as a “real class act“. In this case, it means the person is TERRIBLE.
So, when someone says, “Wow, that guy is a real class act” they mean, that guy has a bunch of terrible characteristics.
It’s important to pay attention to when this term “class act” is used. If the person is really great, then it’s a positive meaning. Just remember that most of the time it’s used in a joking way to refer to someone with those negative qualities.
Ideas from: http://www.speak-fluent-english.com/