It depends on where you live. Some states have sales tax on digital purchases and services. If you are in one of those states then yes, you will need more in your account than the purchase price. I am also not sure if the account you are using has any fees or "ping" charges. Sometimes banks and credit cards do a $1 temp charge to validate the card before the real purchase. That is always refunded to you in a few days, but it would mean the card needs enough on it to cover the ping (or any service fees).