Baby black Arowana's are seasonal, and the season is just ending. They usually become available around April for good prices, although of course there are risks with an egg-sack Arowana or others that are very small.
When you're talking 6"+, the price starts to skyrocket. $250 as a price seems high unless the Arowana is fairly large. $150-$180 seems to be the norm for stable, medium-sized specimens.