The original sentence (grammatically correct) is

He who is happy and cheapest is the richest.

The word happiness is plural.

This is a famous philosophical saying of American philosopher henry david thoreau (18 17- 1862).

Sentence meaning:

He who can seek happiness everywhere is the richest man.

Explanation:

He who enjoys happiness at a low price is the richest. Because sometimes happiness doesn't even cost money. Look at the cool breeze and flowing water. If you know how to get these happiness, you don't need to spend money. He who knows how to seek happiness is the richest man in his heart.