bitinstant exchange rate?

[wrenchmonkey]

I'm trying to figure out how to purchase a specific amount of bitcoins, and trying to figure out what exchange rate they're using when I do so. If I put in that I want to transfer $400 USD to my bitcoin wallet, they say that they'll transfer $384 USD into my wallet. WTF? How do I know how many BITCOINS they're going to put into my wallet? How is this calculated? I can't find it anywhere on their site.