He didn't buy the jean because it is too expensive.

Terry,

"Jeans" is one of those words that, in English, is always plural.  The verb (singular or plural) depends on how "jeans" is being used in the sentence.

1. The jeans are too expensive.  Plural verb are.
2. A pair of jeans is expensive.  Singular verb is needs to agree with pair.
3. Three pairs of jeans are expensive.

I hope this helped.  : )

He didn't buy the jeans because they were too expensive.

(This correctly conveys that the price of the jeans was too high, and he didn't buy a pair because he felt "they", as a collective noun, were too expensive.)