Whenever you message someone, you will get charged for every message you send on a per message basis. Every message in the chats is considered to be 160 or less characters. If you send a message of 160 characters or less, you will pay for only one message at the rate per message shown in the other user profile.
Whenever you cross the 160 characters limit, you will pay for each additional message you decide to write when those get answered. So for example if you send a message with 481 characters, you will pay for the price of 3 messages as you crossed 3 times 160 characters (3x160=480 which is one less characters that the one you have sent in this particular example).
The person answering your messages can then decide to charge the discussion at anytime once the minimum invoice amount has been reached and bring back the balance of the conversation to 0 between you two.