By default, you keep 65% of the messaging price and then a payout share is applied for messaging which is based on response rate. So, the faster you reply, the more you will make!
For example, if you charge $1 per message (100 credits), by default you would get 65% of that $1, which is $0.65. However, if you are online when you receive a message from a fan and answer within 15 minutes, you will get a 125% increase (0.65 X 125% = $0.81). If you get that same message while you are offline and you answer within 24 hours, you will get a 125% increase (0.65 X 125% = $0.81).
To make things easier, every time you change the message price, the prices under the Earnings column will automatically adjust so you can see exactly how much you will make per message depending on your response rate.