You're using Bitcoin... so use "Bitcoin" values... don't attempt to use "fiat" aka "dollar" values... you'll end up in trouble. The value of Bitcoin as it relates to the USD is quite volatile... open up a browser window on any exchange that trades BTC/USD and you'll see what I mean
Even though you put in $ amounts, b.info is using BTC values for the transactions (it has to, it is sending BTC after all... not USD$!)... There are likely to be exchange rate issues and fluctuations in currency value.