$600 is what has been paid for years. I can give you a list of companies that paid $600 for a bg shoot. Think about it this way, do you want to do a scene that is payroll for $900. After they take out around $200-$250 in taxes from your $900. Your now down to $700-$650, then you have to pay your 10% to your agent, or more if you don't have a car.
You are now down to $540 for a bg scene, you just lost $60

. If you can hold to your prices do so, but when the bills are due, the decision should be up to you to decide.