The OP calculated $0.50 / $0.0008 per message to get 625 messages, based on "You’ll receive your first invoice under the new plan on April 1 if your amount due is greater than $0.50. According to your usage last month, your invoice under the new price per message of $0.0008"
That's not quite how it works out though. Mailgun rolls over the balance till the balance hits $0.50 and when the balance hits $0.50, they cut an invoice for that amount.