Remember the transactions fees too.
It's WalletBit that sets the conversion rate. Wait, I'm not sure of this.
For what it's worth, here's how Walletbit does the instant exchange:
Exchange of your bitcoins to US dollars on Mt.Gox will be carried out by WalletBit to clear out your accumulated balance. WalletBit will deduct our processing fee from each merchant payout. Our fee for this is 0.89% + a flat fee of 0.05 BTC to pay miners.
sourceThat should help you calculate it, but maybe Avalon eats the fee.
Maybe you can take a loan for a few hours?