For a definite price you should check out Kelly's Blue Book or a NADA guide. Without knowing more about the car it could be worth anywhere from $4,550 to $14,750.
According to Kelly Blue Book, a 2004 Accord with 30K miles is worth anywhere between $8,000 - $12,000. The actual value depends on the model and its condition. A 2-Door with limited options in fair condition will be at the low end, while a fully loaded 4-door in great condition will be worth the most.