Translation:He will not download the movie because it is expensive.
"dear" is usually used with a qualifier - "too dear" or "very dear". "He will not download the movie because it is dear" sounds a little bit strange to my ears, though I would understand it. Change it "too dear", and it sounds perfectly normal to me.
"Dear" is also not typically used as a synonym for "expensive" in American English.
But that's all a discussion about English, not Irish - it's worth pointing out that the FGB entry for costasach does not give "dear" as a translation: www.teanglann.ie/en/fgb/costasach