“Why did you buy this huge bag of oranges?”
“They were selling them dirt cheap outside the supermarket!”
“Dirt cheap” is very cheap, as cheap as dirt. Dirt is everywhere, so it’s basically free. We can compare something that is extremely cheap to dirt, which costs nothing. For example, when a store is going out of business, they want to get rid of all their stock, so they might sell it dirt cheap. Something can be a good deal without being cheap enough to qualify as “dirt cheap”.
「なんでこんなにたくさんのオレンジを買ったの？」
「スーパーの外で大安売りしてたんだ！」
Dirt cheap は、とても安い、土みたいに安いという意味です。土はどこにでもあり、基本的にタダです。とても安い物を、お金がまったくかからない「土 dirt」に例えることができます。例えば、店が閉店するときに、商品を全部売ってしまう為に大特価で売ることがあります。掘り出し物もありますが dirt cheapとは限りません。
