Uhm, being multi platform DOES add to the development costs overall, just not significantly usually. It mainly increases the potential income, in theory, but that's not always the case. Gundam Masou isn't multi platform, and looks unlikely to hit 500k for a long time worldwide. The multiplatform Virtua Fighter 5 will break 500k total sales worldwide eventually, but it's on the slow boat back to china in that regard. :) Armored Core 4, worldwide, and between 2 consoles doesn't look to hit 500k WW sales anytime soon from what we've seen of the sales.
Nobody is crazy enough to accuse me of being sane.