"Justiça é caro."
"Justiça é cara." sounds more natural. But I believe "Justiça é caro" can also be right.
If "justiça" was the object of a verb (like "fazer justiça"), the adjective would be in the masculine form (as verbs are genderless). So I believe the verb can be implicit there. "Fazer justiça é caro", ou "aplicar a justiça é caro".
The Michaelis online dictionary has a second definition of "caro" (the first is "highly valued" or "expensive") which seems to fit here, it is: "2. difficult, hard earned, requiring sacrifices or considerable expense".
More chat about this sentence here: http://www.duolingo.com/comment/222855